Changes in Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Other Comprehensive Income (Loss) Net of Tax, Period Change [Abstract]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	$ 69	$ (81)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(15)	13
Total other comprehensive income (loss)	54	(69)
Accumulated Other Comprehensive Income (Loss), Net of Tax	87	(128)	$ 33	$ (59)
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Gain (Loss) Arising During Period, Net of Tax	14	10
Gain (Loss) on Derivative Used in Net Investment Hedge, after Tax	(8)	0
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax	6	10
Accumulated Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Net of Tax	(46)	(22)	(53)	(32)
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), before Reclassification, after Tax	56	(91)
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, after Tax	(7)	12
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax	0	1
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification and Tax	49	(80)
Other Comprehensive Income (Loss), Securities, Available-for-sale, Adjustment, before Reclassification Adjustments and Tax	0	0
AOCI, Cash Flow Hedge, Cumulative Gain (Loss), after Tax	160	(79)	111	1
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, after Tax	0	0
Accumulated Other Comprehensive Income (Loss), Securities, Available-for-sale, Adjustment, after Tax	0	0	0	(1)
Other Comprehensive (Income) Loss, Defined Benefit Plan, before Reclassification Adjustment, after Tax	1	0
Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, after Tax	0	0
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	(1)	0
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax	$ (26)	$ (27)	$ (25)	$ (27)