Fair Value Measurements of Assets and Liabilities on Recurring Basis (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Restricted funds	$ 29	$ 24
Rabbi trust investments	13	12
Deposits	3	3
Mark-to-market derivative assets	25	28
Other investments	2	1
Total assets	72	68
Deferred compensation obligations	15	13
Total liabilities	15	13
Total net assets (liabilities)	57	55
Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Restricted funds	29	24
Rabbi trust investments	13	12
Deposits	3	3
Other investments	2	1
Total assets	47	40
Deferred compensation obligations	15	13
Total liabilities	15	13
Total net assets (liabilities)	32	27
Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Mark-to-market derivative assets	25	28
Total assets	25	28
Total net assets (liabilities)	$ 25	$ 28