Accumulated Other Comprehensive Income - Reclassification (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total gains on cash flow hedges	$ 15	$ 5
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total gains on cash flow hedges	(18)	(15)
Amortization of defined benefit plan items
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total gains on cash flow hedges	$ 33	$ 20