Quarterly report pursuant to Section 13 or 15(d)

Fair Value Measurements - Fair Value Measurements by Level for Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details)

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Fair Value Measurements - Fair Value Measurements by Level for Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($)
$ in Millions
Mar. 31, 2021
Dec. 31, 2020
Assets    
Cash and cash equivalents $ 9,627 $ 10,800
Investments:    
Equity securities 319 318
Total assets at fair value 24,579 25,287
U.S. Treasury securities and obligations of U.S. government corporations and agencies    
Investments:    
Assets measured at fair value 517 911
Corporate securities    
Investments:    
Assets measured at fair value 7,062 6,814
Municipal securities    
Investments:    
Assets measured at fair value 3,205 3,097
Short-term time deposits    
Investments:    
Assets measured at fair value 82 53
Asset-backed securities    
Investments:    
Assets measured at fair value 1,207 1,164
Residential mortgage-backed securities    
Investments:    
Assets measured at fair value 1,078 1,095
Commercial mortgage-backed securities    
Investments:    
Assets measured at fair value 801 773
Investments available-for-sale    
Investments:    
Assets measured at fair value 13,839 13,427
Investments available-for-sale | U.S. Treasury securities and obligations of U.S. government corporations and agencies    
Investments:    
Assets measured at fair value 145 165
Investments available-for-sale | Corporate securities    
Investments:    
Assets measured at fair value 7,035 6,789
Investments available-for-sale | Municipal securities    
Investments:    
Assets measured at fair value 3,172 3,070
Investments available-for-sale | Short-term time deposits    
Investments:    
Assets measured at fair value 82 53
Investments available-for-sale | Asset-backed securities    
Investments:    
Assets measured at fair value 1,207 1,164
Investments available-for-sale | Residential mortgage-backed securities    
Investments:    
Assets measured at fair value 1,078 1,095
Investments available-for-sale | Commercial mortgage-backed securities    
Investments:    
Assets measured at fair value 801 773
Restricted deposits    
Investments:    
Assets measured at fair value 1,113 1,060
Restricted deposits | U.S. Treasury securities and obligations of U.S. government corporations and agencies    
Investments:    
Assets measured at fair value 372 746
Restricted deposits | Corporate securities    
Investments:    
Assets measured at fair value 27 25
Restricted deposits | Municipal securities    
Investments:    
Assets measured at fair value 33 27
Restricted deposits | Cash and cash equivalents    
Investments:    
Assets measured at fair value 677 157
Restricted deposits | Certificates of deposit    
Investments:    
Assets measured at fair value 4 105
Level I    
Assets    
Cash and cash equivalents 9,627 10,800
Investments:    
Equity securities 317 316
Total assets at fair value 11,138 12,184
Level I | Investments available-for-sale    
Investments:    
Assets measured at fair value 462 481
Level I | Investments available-for-sale | U.S. Treasury securities and obligations of U.S. government corporations and agencies    
Investments:    
Assets measured at fair value 145 165
Level I | Investments available-for-sale | Corporate securities    
Investments:    
Assets measured at fair value 0 0
Level I | Investments available-for-sale | Municipal securities    
Investments:    
Assets measured at fair value 0 0
Level I | Investments available-for-sale | Short-term time deposits    
Investments:    
Assets measured at fair value 0 0
Level I | Investments available-for-sale | Asset-backed securities    
Investments:    
Assets measured at fair value 0 0
Level I | Investments available-for-sale | Residential mortgage-backed securities    
Investments:    
Assets measured at fair value 0 0
Level I | Investments available-for-sale | Commercial mortgage-backed securities    
Investments:    
Assets measured at fair value 0 0
Level I | Restricted deposits    
Investments:    
Assets measured at fair value 1,049 903
Level I | Restricted deposits | U.S. Treasury securities and obligations of U.S. government corporations and agencies    
Investments:    
Assets measured at fair value 372 746
Level I | Restricted deposits | Corporate securities    
Investments:    
Assets measured at fair value 0 0
Level I | Restricted deposits | Municipal securities    
Investments:    
Assets measured at fair value 0 0
Level I | Restricted deposits | Cash and cash equivalents    
Investments:    
Assets measured at fair value 677 157
Level I | Restricted deposits | Certificates of deposit    
Investments:    
Assets measured at fair value 0 0
Level II    
Assets    
Cash and cash equivalents 0 0
Investments:    
Equity securities 2 2
Total assets at fair value 13,441 13,103
Level II | Investments available-for-sale    
Investments:    
Assets measured at fair value 13,377 12,946
Level II | Investments available-for-sale | U.S. Treasury securities and obligations of U.S. government corporations and agencies    
Investments:    
Assets measured at fair value 0 0
Level II | Investments available-for-sale | Corporate securities    
Investments:    
Assets measured at fair value 7,035 6,789
Level II | Investments available-for-sale | Municipal securities    
Investments:    
Assets measured at fair value 3,172 3,070
Level II | Investments available-for-sale | Short-term time deposits    
Investments:    
Assets measured at fair value 82 53
Level II | Investments available-for-sale | Asset-backed securities    
Investments:    
Assets measured at fair value 1,207 1,164
Level II | Investments available-for-sale | Residential mortgage-backed securities    
Investments:    
Assets measured at fair value 1,078 1,095
Level II | Investments available-for-sale | Commercial mortgage-backed securities    
Investments:    
Assets measured at fair value 801 773
Level II | Restricted deposits    
Investments:    
Assets measured at fair value 64 157
Level II | Restricted deposits | U.S. Treasury securities and obligations of U.S. government corporations and agencies    
Investments:    
Assets measured at fair value 0 0
Level II | Restricted deposits | Corporate securities    
Investments:    
Assets measured at fair value 27 25
Level II | Restricted deposits | Municipal securities    
Investments:    
Assets measured at fair value 33 27
Level II | Restricted deposits | Cash and cash equivalents    
Investments:    
Assets measured at fair value 0 0
Level II | Restricted deposits | Certificates of deposit    
Investments:    
Assets measured at fair value 4 105
Level III    
Assets    
Cash and cash equivalents 0 0
Investments:    
Equity securities 0 0
Total assets at fair value 0 0
Level III | Investments available-for-sale    
Investments:    
Assets measured at fair value 0 0
Level III | Investments available-for-sale | U.S. Treasury securities and obligations of U.S. government corporations and agencies    
Investments:    
Assets measured at fair value 0 0
Level III | Investments available-for-sale | Corporate securities    
Investments:    
Assets measured at fair value 0 0
Level III | Investments available-for-sale | Municipal securities    
Investments:    
Assets measured at fair value 0 0
Level III | Investments available-for-sale | Short-term time deposits    
Investments:    
Assets measured at fair value 0 0
Level III | Investments available-for-sale | Asset-backed securities    
Investments:    
Assets measured at fair value 0 0
Level III | Investments available-for-sale | Residential mortgage-backed securities    
Investments:    
Assets measured at fair value 0 0
Level III | Investments available-for-sale | Commercial mortgage-backed securities    
Investments:    
Assets measured at fair value 0 0
Level III | Restricted deposits    
Investments:    
Assets measured at fair value 0 0
Level III | Restricted deposits | U.S. Treasury securities and obligations of U.S. government corporations and agencies    
Investments:    
Assets measured at fair value 0 0
Level III | Restricted deposits | Corporate securities    
Investments:    
Assets measured at fair value 0 0
Level III | Restricted deposits | Municipal securities    
Investments:    
Assets measured at fair value 0 0
Level III | Restricted deposits | Cash and cash equivalents    
Investments:    
Assets measured at fair value 0 0
Level III | Restricted deposits | Certificates of deposit    
Investments:    
Assets measured at fair value $ 0 $ 0