Fair Value Measurements Fair Value Measurements - Accumulated Other Comprehensive Income/Loss (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	$ (37)	$ 15	$ (24)	$ 6
OCI, before Reclassifications, Net of Tax, Attributable to Parent	(13)	9
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax	0	3
Other Comprehensive Income (Loss), Net of Tax	(15)	8
Designated as Hedging Instrument [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax	(30)	4	(13)	0
OCI, before Reclassifications, Net of Tax, Attributable to Parent	(17)	4
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax	(17)	4
Accumulated Foreign Currency Adjustment Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Net of Tax	(7)	10	(9)	9
OCI, before Reclassifications, Net of Tax, Attributable to Parent	2	1
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax	2	1
Other [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	0	1	$ (2)	$ (3)
OCI, before Reclassifications, Net of Tax, Attributable to Parent	2	4
Other Comprehensive Income, Other, Net of Tax	2	4
AOCI Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), Net of Tax	$ (13)	$ 9