Fair Value Measurements - Accumulated Other Comprehensive Income/Loss (Details) - USD ($) $ in Millions	3 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 2,808	$ 3,122	$ 3,338	$ 3,306	$ 3,302	$ 3,297
Other comprehensive income/(loss) before reclassifications	(12)	(41)	(13)	14	(11)	9
Amounts reclassified from accumulated other comprehensive loss	3	1
Total other comprehensive income/(loss), net of tax	(9)	(40)	(13)	14	(11)	9
Ending Balance	2,467	2,808	3,122	3,167	3,306	3,302
Unrealized Net Gains/(Losses) on Derivative Instruments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(65)	(30)	(13)	13	4	0
Other comprehensive income/(loss) before reclassifications	(6)	(36)	(17)	11	9	4
Amounts reclassified from accumulated other comprehensive loss	3	1
Total other comprehensive income/(loss), net of tax	(3)	(35)	(17)	11	9	4
Ending Balance	(68)	(65)	(30)	24	13	4
Foreign Currency Translation Adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(12)	(7)	(9)	(7)	10	9
Other comprehensive income/(loss) before reclassifications	(6)	(5)	2	3	(17)	1
Amounts reclassified from accumulated other comprehensive loss	0	0
Total other comprehensive income/(loss), net of tax	(6)	(5)	2	3	(17)	1
Ending Balance	(18)	(12)	(7)	(4)	(7)	10
Other
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	0	0	(2)	(2)	1	(3)
Other comprehensive income/(loss) before reclassifications	0	0	2	0	(3)	4
Amounts reclassified from accumulated other comprehensive loss	0	0
Total other comprehensive income/(loss), net of tax	0	0	2	0	(3)	4
Ending Balance	0	0	0	(2)	(2)	1
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(77)	(37)	(24)	4	15	6
Ending Balance	$ (86)	$ (77)	$ (37)	$ 18	$ 4	$ 15