Fair Value Measurements of Assets and Liabilities on Recurring Basis (Details) - USD ($)
$ in Millions
Sep. 30, 2016
Dec. 31, 2015
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Restricted funds $ 28 $ 27
Rabbi trust investments 13 12
Deposits 4 1
Mark-to-market derivative asset 1 2
Other investments 4 4
Total assets 50 46
Deferred compensation obligation 13 11
Mark-to-market derivative liabilities 26 1
Total liabilities 39 12
Total net assets (liabilities) 11 34
Level 1    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Restricted funds 28 27
Rabbi trust investments 13 12
Deposits 4 1
Other investments 4 4
Total assets 49 44
Deferred compensation obligation 13 11
Total liabilities 13 11
Total net assets (liabilities) 36 33
Level 2    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Mark-to-market derivative asset 1 2
Total assets 1 2
Mark-to-market derivative liabilities 26 1
Total liabilities 26 1
Total net assets (liabilities) $ (25) $ 1