Accumulated Other Comprehensive Loss (Rollforward) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016
Other Comprehensive Income (Loss), Securities, Available-for-sale, Adjustment, after Tax [Abstract]
Accumulated other comprehensive income (loss), investment securities, adjustment, after tax	$ (731)	$ (131)	$ (731)	$ (131)	$ (186)	$ (173)	$ (273)
Other comprehensive income (loss), investment securities, adjustment, after tax	(133)	46	(545)	142
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax [Abstract]
Accumulated other comprehensive income (loss), foreign currency translation adjustment, net of tax	19	16	19	16	20	16	14
Other comprehensive income (loss), foreign currency translation adjustment, net of tax			(1)	2
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification and Tax [Abstract]
Accumulated other comprehensive income (loss), cumulative changes in net gain (loss) from cash flow hedges, net of tax	28	9	28	9	11	11	8
Other comprehensive income (loss), cash flow hedges, net change, after tax	0	0	17	1
Other Comprehensive Income (Loss), Defined Benefit Plan, Net of Tax [Abstract]
Accumulated other comprehensive income (loss), defined benefit pension plans, after tax	97	91	97	91	95	89	90
Other comprehensive (income) loss, defined benefit plan, after reclassification adjustment, after tax			2	1
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Total accumulated other comprehensive loss, net of tax	781	197	781	197	250	$ 235	$ 341
Other comprehensive income (loss), net of tax	$ (133)	$ 48	$ (531)	$ 144
Accounting Standards Update 2016-01
Other Comprehensive Income (Loss), Securities, Available-for-sale, Adjustment, after Tax [Abstract]
Accumulated other comprehensive income (loss), investment securities, adjustment, after tax					27
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax [Abstract]
Accumulated other comprehensive income (loss), foreign currency translation adjustment, net of tax					0
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification and Tax [Abstract]
Accumulated other comprehensive income (loss), cumulative changes in net gain (loss) from cash flow hedges, net of tax					0
Other Comprehensive Income (Loss), Defined Benefit Plan, Net of Tax [Abstract]
Accumulated other comprehensive income (loss), defined benefit pension plans, after tax					0
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Total accumulated other comprehensive loss, net of tax					(27)
Accounting Standards Update 2018-02
Other Comprehensive Income (Loss), Securities, Available-for-sale, Adjustment, after Tax [Abstract]
Accumulated other comprehensive income (loss), investment securities, adjustment, after tax					(40)
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax [Abstract]
Accumulated other comprehensive income (loss), foreign currency translation adjustment, net of tax					4
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification and Tax [Abstract]
Accumulated other comprehensive income (loss), cumulative changes in net gain (loss) from cash flow hedges, net of tax					0
Other Comprehensive Income (Loss), Defined Benefit Plan, Net of Tax [Abstract]
Accumulated other comprehensive income (loss), defined benefit pension plans, after tax					6
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Total accumulated other comprehensive loss, net of tax					$ 42