Accumulated Other Comprehensive Income - Changes in Accumulated Balances of Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accumulated Other Comprehensive Income (Loss), Tax
Accumulated other comprehensive income (loss), estimated tax (expense) benefit, beginning balance	$ 57	$ 13	$ 41	$ 1
Other comprehensive income (loss) before reclassifications, tax	(24)	20	(2)	32
Less: Amount of gain (loss) reclassified from accumulated other comprehensive income, tax	5	0	11	0
Net current period other comprehensive income, tax	(29)	20	(13)	32
Accumulated other comprehensive income (loss), estimated tax (expense) benefit, ending balance	28	33	28	33
Accumulated Other Comprehensive Income (Loss), Net of Tax
Beginning balance			8,049
Less: Amount of gain (loss) reclassified from accumulated other comprehensive income	(16)	20	(39)	52
Other comprehensive income, net of tax	(219)	272	(136)	561
Ending balance	7,146		7,146
Unrealized Gains (Losses) on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Before Tax
Accumulated other comprehensive income (loss), before tax, beginning balance	(86)	20	(57)	54
Other comprehensive income (loss), before reclassifications, before tax	116	(49)	59	(63)
Less: Amount of gain (loss) reclassified from accumulated other comprehensive income, before tax	(21)	15	(49)	35
Net current period other comprehensive income, before tax	137	(64)	108	(98)
Accumulated other comprehensive income (loss), before tax, ending balance	51	(44)	51	(44)
Unrealized Gains on Investments
Accumulated Other Comprehensive Income (Loss), Before Tax
Accumulated other comprehensive income (loss), before tax, beginning balance	(51)	31	(15)	51
Other comprehensive income (loss), before reclassifications, before tax	(2)	(24)	(39)	(32)
Less: Amount of gain (loss) reclassified from accumulated other comprehensive income, before tax	0	5	(1)	17
Net current period other comprehensive income, before tax	(2)	(29)	(38)	(49)
Accumulated other comprehensive income (loss), before tax, ending balance	(53)	2	(53)	2
Foreign Currency Translation
Accumulated Other Comprehensive Income (Loss), Before Tax
Accumulated other comprehensive income (loss), before tax, beginning balance	880	101	748	(230)
Other comprehensive income (loss), before reclassifications, before tax	(325)	345	(193)	676
Less: Amount of gain (loss) reclassified from accumulated other comprehensive income, before tax	0	0	0	0
Net current period other comprehensive income, before tax	(325)	345	(193)	676
Accumulated other comprehensive income (loss), before tax, ending balance	555	446	555	446
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax
Beginning balance	800	165	717	(124)
Other comprehensive income (loss) before reclassifications	(235)	292	(175)	613
Less: Amount of gain (loss) reclassified from accumulated other comprehensive income	(16)	20	(39)	52
Other comprehensive income, net of tax	(219)	272	(136)	561
Ending balance	$ 581	$ 437	$ 581	$ 437