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Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Restricted Cash Equivalents, Noncurrent $ 9 $ 7
Recurring Basis [Member]    
Assets:    
Cash equivalents 19 61
Restricted cash equivalents(a) 21 31
Interest rate contracts 44 15
Total assets 84 107
Liabilities:    
Interest rate contracts 125 44
Foreign currency contracts 1 3
Total liabilities 126 47
Recurring Basis [Member] | Level 1 [Member]    
Assets:    
Cash equivalents 19 61
Restricted cash equivalents(a) 21 31
Interest rate contracts 0 0
Total assets 40 92
Liabilities:    
Interest rate contracts 0 0
Foreign currency contracts 0 0
Total liabilities 0 0
Recurring Basis [Member] | Level 2 [Member]    
Assets:    
Cash equivalents 0 0
Restricted cash equivalents(a) 0 0
Interest rate contracts 44 15
Total assets 44 15
Liabilities:    
Interest rate contracts 125 44
Foreign currency contracts 1 3
Total liabilities $ 126 $ 47