Accumulated Other Comprehensive Loss (Rollforward) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax [Abstract]
Beginning Balance Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	$ (273)	$ (159)	$ (21)
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	100	(114)	(138)
Ending Balance Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	(173)	(273)	(159)
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax [Abstract]
Beginning Balance Accumulated Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Net of Tax	14	9	36
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax	2	5	(27)
Ending Balance Accumulated Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Net of Tax	16	14	9
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax [Abstract]
Beginning Balance Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax	8	8	7
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	3	0	1
Ending Balance Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax	11	8	8
Other Comprehensive Income (Loss), Defined Benefit Plan, Net of Tax [Abstract]
Beginning Balance Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax	(90)	(89)	(88)
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	(1)	1	1
Ending Balance Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax	(89)	(90)	(89)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Accumulated Other Comprehensive Income (Loss), Net of Tax	235	341	231	$ 66
Other Comprehensive Income (Loss), Net of Tax	106	(110)	(165)
Accumulated Other Comprehensive Income (Loss), Net of Tax	$ (235)	$ (341)	$ (231)	$ (66)