Accumulated Other Comprehensive Income - Changes in Accumulated Balances of Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Accumulated Other Comprehensive Income (Loss), Tax
Accumulated other comprehensive income (loss), estimated tax (expense) benefit, beginning balance	$ 23	$ 57	$ 24	$ 41
Other comprehensive income (loss) before reclassifications, tax	(10)	(24)	(15)	(2)
Less: Amount of gain (loss) reclassified from accumulated other comprehensive income, tax	(3)	5	(7)	11
Net current period other comprehensive income (loss), tax	(7)	(29)	(8)	(13)
Accumulated other comprehensive income (loss), estimated tax (expense) benefit, ending balance	16	28	16	28
Accumulated Other Comprehensive Income (Loss), Net of Tax
Beginning balance			6,281
Other comprehensive income/(loss), net of tax	0	(219)	(34)	(136)
Ending balance	4,106	7,146	4,106	7,146
Unrealized Gains (Losses) on Derivative Instruments
Accumulated Other Comprehensive Income (Loss), Before Tax
Accumulated other comprehensive income (loss), before tax, beginning balance	27	(86)	68	(57)
Other comprehensive income (loss), before reclassifications, before tax	25	116	4	59
Less: Amount of gain (loss) reclassified from accumulated other comprehensive income, before tax	14	(21)	34	(49)
Net current period other comprehensive income (loss), before tax	11	137	(30)	108
Accumulated other comprehensive income (loss), before tax, ending balance	38	51	38	51
Unrealized Gains (Losses) on Investments
Accumulated Other Comprehensive Income (Loss), Before Tax
Accumulated other comprehensive income (loss), before tax, beginning balance	(16)	(51)	(56)	(15)
Other comprehensive income (loss), before reclassifications, before tax	16	(2)	56	(39)
Less: Amount of gain (loss) reclassified from accumulated other comprehensive income, before tax	0	0	0	(1)
Net current period other comprehensive income (loss), before tax	16	(2)	56	(38)
Accumulated other comprehensive income (loss), before tax, ending balance	0	(53)	0	(53)
Foreign Currency Translation
Accumulated Other Comprehensive Income (Loss), Before Tax
Accumulated other comprehensive income (loss), before tax, beginning balance	430	880	462	748
Other comprehensive income (loss), before reclassifications, before tax	(20)	(325)	(52)	(193)
Less: Amount of gain (loss) reclassified from accumulated other comprehensive income, before tax	0	0	0	0
Net current period other comprehensive income (loss), before tax	(20)	(325)	(52)	(193)
Accumulated other comprehensive income (loss), before tax, ending balance	410	555	410	555
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax
Beginning balance	464	800	498	717
Other comprehensive income (loss) before reclassifications	11	(235)	(7)	(175)
Less: Amount of gain (loss) reclassified from accumulated other comprehensive income	11	(16)	27	(39)
Other comprehensive income/(loss), net of tax	0	(219)	(34)	(136)
Ending balance	$ 464	$ 581	$ 464	$ 581