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
Jun. 30, 2021
Dec. 31, 2020
Assets    
Cash and cash equivalents $ 11,018 $ 10,800
Investments:    
Equity securities 317 318
Total assets at fair value 26,081 25,287
U.S. Treasury securities and obligations of U.S. government corporations and agencies    
Investments:    
Assets measured at fair value 605 911
Corporate securities    
Investments:    
Assets measured at fair value 7,417 6,814
Municipal securities    
Investments:    
Assets measured at fair value 3,455 3,097
Short-term time deposits    
Investments:    
Assets measured at fair value 82 53
Asset-backed securities    
Investments:    
Assets measured at fair value 1,184 1,164
Residential mortgage-backed securities    
Investments:    
Assets measured at fair value 996 1,095
Commercial mortgage-backed securities    
Investments:    
Assets measured at fair value 826 773
Investments available-for-sale    
Investments:    
Assets measured at fair value 13,951 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 136 165
Investments available-for-sale | Corporate securities    
Investments:    
Assets measured at fair value 7,387 6,789
Investments available-for-sale | Municipal securities    
Investments:    
Assets measured at fair value 3,023 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 1,184 1,164
Investments available-for-sale | Residential mortgage-backed securities    
Investments:    
Assets measured at fair value 996 1,095
Investments available-for-sale | Commercial mortgage-backed securities    
Investments:    
Assets measured at fair value 826 773
Restricted deposits    
Investments:    
Assets measured at fair value 1,112 1,060
Restricted deposits | U.S. Treasury securities and obligations of U.S. government corporations and agencies    
Investments:    
Assets measured at fair value 469 746
Restricted deposits | Corporate securities    
Investments:    
Assets measured at fair value 30 25
Restricted deposits | Municipal securities    
Investments:    
Assets measured at fair value 432 27
Restricted deposits | Cash and cash equivalents    
Investments:    
Assets measured at fair value 177 157
Restricted deposits | Certificates of deposit    
Investments:    
Assets measured at fair value 4 105
Level I    
Assets    
Cash and cash equivalents 11,018 10,800
Investments:    
Equity securities 315 316
Total assets at fair value 12,115 12,184
Level I | Investments available-for-sale    
Investments:    
Assets measured at fair value 451 481
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 136 165
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 646 903
Level I | Restricted deposits | U.S. Treasury securities and obligations of U.S. government corporations and agencies    
Investments:    
Assets measured at fair value 469 746
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 177 157
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 13,966 13,103
Level II | Investments available-for-sale    
Investments:    
Assets measured at fair value 13,500 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 7,387 6,789
Level II | Investments available-for-sale | Municipal securities    
Investments:    
Assets measured at fair value 3,023 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 1,184 1,164
Level II | Investments available-for-sale | Residential mortgage-backed securities    
Investments:    
Assets measured at fair value 996 1,095
Level II | Investments available-for-sale | Commercial mortgage-backed securities    
Investments:    
Assets measured at fair value 826 773
Level II | Restricted deposits    
Investments:    
Assets measured at fair value 466 157
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 30 25
Level II | Restricted deposits | Municipal securities    
Investments:    
Assets measured at fair value 432 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