Financial Instruments - Terminated Swaps (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Amortization of fair value adjustments for terminated swaps	$ 13	$ 19	$ 22
Expected net decrease to interest expense in future years through 2022	$ 4