Accumulated Other Comprehensive Income - Changes in Accumulated Balances of Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Accumulated Other Comprehensive Income (Loss), Tax
Accumulated other comprehensive income (loss), estimated tax (expense) benefit, beginning balance	$ 16	$ 28	$ 24	$ 41
Other comprehensive income (loss) before reclassifications, tax	(12)	(1)	(27)	(3)
Less: Amount of gain (loss) reclassified from accumulated other comprehensive income, tax	(4)	(3)	(11)	8
Net current period other comprehensive income (loss), tax	(8)	2	(16)	(11)
Accumulated other comprehensive income (loss), estimated tax (expense) benefit, ending balance	8	30	8	30
Accumulated Other Comprehensive Income (Loss), Net of Tax
Beginning balance			6,281
Other comprehensive income (loss) before reclassifications	(88)	(22)	(95)	(197)
Less: Amount of gain (loss) reclassified from accumulated other comprehensive income	15	9	42	(30)
Other comprehensive income (loss), net of tax	(103)	(31)	(137)	(167)
Ending balance	3,270	6,929	3,270	6,929
Unrealized Gains (Losses) on Derivative Instruments
Accumulated Other Comprehensive Income (Loss), Before Tax
Accumulated other comprehensive income (loss), before tax, beginning balance	38	51	68	(57)
Other comprehensive income (loss), before reclassifications, before tax	54	(4)	58	55
Less: Amount of gain (loss) reclassified from accumulated other comprehensive income, before tax	19	12	53	(37)
Net current period other comprehensive income (loss), before tax	35	(16)	5	92
Accumulated other comprehensive income (loss), before tax, ending balance	73	35	73	35
Unrealized Gains (Losses) on Investments
Accumulated Other Comprehensive Income (Loss), Before Tax
Accumulated other comprehensive income (loss), before tax, beginning balance	0	(53)	(56)	(15)
Other comprehensive income (loss), before reclassifications, before tax	3	8	59	(31)
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	3	8	59	(30)
Accumulated other comprehensive income (loss), before tax, ending balance	3	(45)	3	(45)
Foreign Currency Translation
Accumulated Other Comprehensive Income (Loss), Before Tax
Accumulated other comprehensive income (loss), before tax, beginning balance	410	555	462	748
Other comprehensive income (loss), before reclassifications, before tax	(133)	(25)	(185)	(218)
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	(133)	(25)	(185)	(218)
Accumulated other comprehensive income (loss), before tax, ending balance	277	530	277	530
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax
Beginning balance	464	581	498	717
Ending balance	$ 361	$ 550	$ 361	$ 550