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, 2015
Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents $ 1,967 $ 1,610
U.S. Treasury securities and obligations of U.S. government corporations and agencies 436 377
Corporate securities 632 556
Municipal securities, General obligation 96 54
Municipal securities, Pre-refunded 4 5
Municipal securities, Revenue 161 102
Municipal securities, Variable rate demand notes 28 14
Asset backed securities 162 180
Mortgage backed securities 78 85
Total investments 1,597 1,373
Cash and cash equivalents 80 79
Certificates of deposit 6 6
U.S. Treasury securities and obligations of U.S. government corporations and agencies 15 15
Total restricted deposits 101 100
Other long term assets: Interest rate swap agreements 10 11
Total assets at fair value 3,675 3,094
Other long term liabilities: Interest rate swap agreements 5  
Total liabilities at fair value 5  
Level I [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 1,967 1,610
U.S. Treasury securities and obligations of U.S. government corporations and agencies 433 360
Corporate securities 0 0
Municipal securities, General obligation 0 0
Municipal securities, Pre-refunded 0 0
Municipal securities, Revenue 0 0
Municipal securities, Variable rate demand notes 0 0
Asset backed securities 0 0
Mortgage backed securities 0 0
Total investments 433 360
Cash and cash equivalents 80 79
Certificates of deposit 6 6
U.S. Treasury securities and obligations of U.S. government corporations and agencies 15 15
Total restricted deposits 101 100
Other long term assets: Interest rate swap agreements 0 0
Total assets at fair value 2,501 2,070
Other long term liabilities: Interest rate swap agreements 0  
Total liabilities at fair value 0  
Level II [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 0 0
U.S. Treasury securities and obligations of U.S. government corporations and agencies 3 17
Corporate securities 632 556
Municipal securities, General obligation 96 54
Municipal securities, Pre-refunded 4 5
Municipal securities, Revenue 161 102
Municipal securities, Variable rate demand notes 28 14
Asset backed securities 162 180
Mortgage backed securities 78 85
Total investments 1,164 1,013
Cash and cash equivalents 0 0
Certificates of deposit 0 0
U.S. Treasury securities and obligations of U.S. government corporations and agencies 0 0
Total restricted deposits 0 0
Other long term assets: Interest rate swap agreements 10 11
Total assets at fair value 1,174 1,024
Other long term liabilities: Interest rate swap agreements 5  
Total liabilities at fair value 5  
Level III [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash and cash equivalents 0 0
U.S. Treasury securities and obligations of U.S. government corporations and agencies 0 0
Corporate securities 0 0
Municipal securities, General obligation 0 0
Municipal securities, Pre-refunded 0 0
Municipal securities, Revenue 0 0
Municipal securities, Variable rate demand notes 0 0
Asset backed securities 0 0
Mortgage backed securities 0 0
Total investments 0 0
Cash and cash equivalents 0 0
Certificates of deposit 0 0
U.S. Treasury securities and obligations of U.S. government corporations and agencies 0 0
Total restricted deposits 0 0
Other long term assets: Interest rate swap agreements 0 0
Total assets at fair value 0 $ 0
Other long term liabilities: Interest rate swap agreements 0  
Total liabilities at fair value $ 0