Accumulated Other Comprehensive Income - Changes in Accumulated Balances of Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Accumulated Other Comprehensive Income (Loss), Tax
Accumulated other comprehensive income (loss), estimated tax (expense) benefit, beginning balance	$ 1	$ (310)
Other comprehensive income (loss) before reclassifications, tax	12	(21)
Less: Amount of gain (loss) reclassified from accumulated other comprehensive income, tax	0	0
Net current period other comprehensive income, tax	12	(21)
Accumulated other comprehensive income (loss), estimated tax (expense) benefit, ending balance	13	(331)
Accumulated Other Comprehensive Income (Loss), Net of Tax
Beginning balance	10,539
Less: Amount of gain (loss) reclassified from accumulated other comprehensive income	32	11
Other comprehensive income (loss), net of tax	289	159
Ending balance	11,595
Unrealized Gains (Losses) on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Before Tax
Accumulated other comprehensive income (loss), before tax, beginning balance	54	36
Other comprehensive income (loss), before reclassifications, before tax	(14)	19
Less: Amount of gain (loss) reclassified from accumulated other comprehensive income, before tax	20	16
Net current period other comprehensive income, before tax	(34)	3
Accumulated other comprehensive income (loss), before tax, ending balance	20	39
Unrealized Gains on Investments
Accumulated Other Comprehensive Income (Loss), Before Tax
Accumulated other comprehensive income (loss), before tax, beginning balance	51	845
Other comprehensive income (loss), before reclassifications, before tax	(8)	18
Less: Amount of gain (loss) reclassified from accumulated other comprehensive income, before tax	12	(5)
Net current period other comprehensive income, before tax	(20)	23
Accumulated other comprehensive income (loss), before tax, ending balance	31	868
Foreign Currency Translation
Accumulated Other Comprehensive Income (Loss), Before Tax
Accumulated other comprehensive income (loss), before tax, beginning balance	(230)	(45)
Other comprehensive income (loss), before reclassifications, before tax	331	154
Less: Amount of gain (loss) reclassified from accumulated other comprehensive income, before tax	0	0
Net current period other comprehensive income, before tax	331	154
Accumulated other comprehensive income (loss), before tax, ending balance	101	109
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax
Beginning balance	(124)	526
Other comprehensive income (loss) before reclassifications	321	170
Less: Amount of gain (loss) reclassified from accumulated other comprehensive income	32	11
Other comprehensive income (loss), net of tax	289	159
Ending balance	$ 165	$ 685