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Financial Instruments (Details) - USD ($)
$ in Millions
Sep. 30, 2018
Dec. 31, 2017
Financial instruments    
Restricted cash and cash equivalents $ 568 $ 456
Long-term debt, including current maturities 9,327 7,396
Carrying amount    
Financial instruments    
Cash and cash equivalents 2,307 2,519
Short-term investments   450
Restricted cash and cash equivalents 568 456
Restricted investments 7 33
Long-term debt, including current maturities 9,327 7,396
Derivative instruments, liabilities 2  
Fair value    
Financial instruments    
Cash and cash equivalents 2,307 2,519
Short-term investments   450
Restricted cash and cash equivalents 568 456
Restricted investments 7 33
Long-term debt, including current maturities 9,949 7,538
Derivative instruments, liabilities 2  
Other current assets    
Financial instruments    
Restricted cash and cash equivalents 561 440
Other assets    
Financial instruments    
Restricted cash and cash equivalents 7 16
Significant other observable inputs    
Financial instruments    
Restricted investments, noncurrent $ 7  
Significant other observable inputs | Fair value    
Financial instruments    
Restricted investments   33
Significant other observable inputs | Other current assets | Fair value    
Financial instruments    
Restricted investments   26
Significant other observable inputs | Other assets | Fair value    
Financial instruments    
Restricted investments   $ 7