Accumulated Other Comprehensive Income - Changes in Accumulated Balances of Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Accumulated Other Comprehensive Income (Loss), Tax
Accumulated other comprehensive income (loss), estimated tax (expense) benefit, beginning balance	$ 13	$ (331)	$ 1	$ (310)
Other comprehensive income (loss) before reclassifications, tax	20	(80)	32	(101)
Less: Amount of gain (loss) reclassified from accumulated other comprehensive income, tax	0	0	0	0
Net current period other comprehensive income, tax	20	(80)	32	(101)
Accumulated other comprehensive income (loss), estimated tax (expense) benefit, ending balance	33	(411)	33	(411)
Accumulated Other Comprehensive Income (Loss), Net of Tax
Beginning balance			10,539
Less: Amount of gain (loss) reclassified from accumulated other comprehensive income	20	(8)	52	3
Other comprehensive income, net of tax	272	144	561	303
Ending balance	11,474		11,474
Unrealized Gains (Losses) on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Before Tax
Accumulated other comprehensive income (loss), before tax, beginning balance	20	39	54	36
Other comprehensive income (loss), before reclassifications, before tax	(49)	50	(63)	69
Less: Amount of gain (loss) reclassified from accumulated other comprehensive income, before tax	15	20	35	36
Net current period other comprehensive income, before tax	(64)	30	(98)	33
Accumulated other comprehensive income (loss), before tax, ending balance	(44)	69	(44)	69
Unrealized Gains on Investments
Accumulated Other Comprehensive Income (Loss), Before Tax
Accumulated other comprehensive income (loss), before tax, beginning balance	31	868	51	845
Other comprehensive income (loss), before reclassifications, before tax	(24)	253	(32)	271
Less: Amount of gain (loss) reclassified from accumulated other comprehensive income, before tax	5	(28)	17	(33)
Net current period other comprehensive income, before tax	(29)	281	(49)	304
Accumulated other comprehensive income (loss), before tax, ending balance	2	1,149	2	1,149
Foreign Currency Translation
Accumulated Other Comprehensive Income (Loss), Before Tax
Accumulated other comprehensive income (loss), before tax, beginning balance	101	109	(230)	(45)
Other comprehensive income (loss), before reclassifications, before tax	345	(87)	676	67
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	345	(87)	676	67
Accumulated other comprehensive income (loss), before tax, ending balance	446	22	446	22
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax
Beginning balance	165	685	(124)	526
Other comprehensive income (loss) before reclassifications	292	136	613	306
Less: Amount of gain (loss) reclassified from accumulated other comprehensive income	20	(8)	52	3
Other comprehensive income, net of tax	272	144	561	303
Ending balance	$ 437	$ 829	$ 437	$ 829