CONDENSED CONSOLIDATED STATEMENT OF COMPREHENSIVE INCOME - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 129	$ 147	$ 193	$ 145	$ 127	$ 139	$ 133	$ 153	$ 614	$ 552	$ 268
Available-for-sale securities:
Unrealized gains (losses), net									137	(5)	(84)
Reclassification into earnings, net									(24)	(33)	(24)
Net change from available-for-sale securities									113	(38)	(108)
Cash flow hedging instruments:
Unrealized losses, net									0	0	(10)
Reclassification into earnings, net									0	0	271
Net change from cash flow hedging instruments									0	0	261
Foreign currency translation:
Foreign currency translation losses, net									0	0	(3)
Reclassification into earnings, net									(2)	0	0
Net change from foreign currency translation									(2)	0	(3)
Other comprehensive income (loss)									111	(38)	150
Comprehensive income									$ 725	$ 514	$ 418