Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss), Tax
Beginning balance, Estimated tax (expense) benefit	$ 41	$ 1
Other comprehensive income (loss) before reclassifications, Estimated tax (expense) benefit	(15)	40
Less: Amount of gain (loss) reclassified from accumulated other comprehensive income, Estimated tax (expense) benefit	2	0
Net current period other comprehensive income (loss), Estimated tax (expense) benefit	(17)	40
Ending balance, Estimated tax (expense) benefit	24	41	$ 1
Accumulated Other Comprehensive Income (Loss), Net of Tax
Stockholders' equity, beginning of period	8,049	10,526
Other comprehensive income (loss), net of tax	(219)	841	(650)
Stockholders' equity, end of period	6,281	8,049	10,526
Unrealized Gains (Losses) on Derivative Instruments
Accumulated Other Comprehensive Income (Loss), Before Tax
Beginning balance, before tax	(57)	54
Other comprehensive income (loss) before reclassifications	117	(104)
Less: Amount of gain (loss) reclassified from AOCI	(8)	7
Net current period other comprehensive income (loss)	125	(111)
Ending balance, before tax	68	(57)	54
Unrealized Gains (Losses) on Investments
Accumulated Other Comprehensive Income (Loss), Before Tax
Beginning balance, before tax	(15)	51
Other comprehensive income (loss) before reclassifications	(42)	(59)
Less: Amount of gain (loss) reclassified from AOCI	(1)	7
Net current period other comprehensive income (loss)	(41)	(66)
Ending balance, before tax	(56)	(15)	51
Foreign Currency Translation
Accumulated Other Comprehensive Income (Loss), Before Tax
Beginning balance, before tax	748	(230)
Other comprehensive income (loss) before reclassifications	(286)	978
Less: Amount of gain (loss) reclassified from AOCI	0	0
Net current period other comprehensive income (loss)	(286)	978
Ending balance, before tax	462	748	(230)
AOCI
Accumulated Other Comprehensive Income (Loss), Net of Tax
Stockholders' equity, beginning of period	717	(124)
Other comprehensive income (loss) before reclassifications, net of tax	(226)	855
Less: Amount of gain (loss) reclassified from accumulated other comprehensive income, net of tax	(7)	14
Other comprehensive income (loss), net of tax	(219)	841
Stockholders' equity, end of period	$ 498	$ 717	$ (124)