Summary of Significant Accounting Policies - Accumulated OCI (loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Change in Accumulated OCI (loss) balances [Roll Forward]
Beginning Balance	$ (128)
Current period change	(2)	$ (53)	$ 48
Ending Balance	(130)	(128)
Qualifying Hedges [Member]
Change in Accumulated OCI (loss) balances [Roll Forward]
Beginning Balance	(41)
Current period change	(7)
Ending Balance	(48)	(41)
Marketable Securities [Member]
Change in Accumulated OCI (loss) balances [Roll Forward]
Beginning Balance	0
Current period change	0
Ending Balance	0	0
Pension and Other Postretirement Benefit Plans [Member]
Change in Accumulated OCI (loss) balances [Roll Forward]
Beginning Balance	(87)
Current period change	5
Ending Balance	$ (82)	$ (87)