Accumulated Other Comprehensive Income (Components of Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other comprehensive income (loss) before tax
Net unrealized gain (loss)	$ 13	$ (44)	$ (477)
Reclassification of net unrealized loss transferred to held to maturity	227	0	0
Other reclassifications included in other revenue	(12)	(4)	0
Change in net unrealized gain (loss) on held to maturity securities:
Reclassification of net unrealized loss transferred from available for sale	(227)	0	0
Amortization of amounts previously recorded upon transfer from available for sale	31	0	0
Other	(11)	1	0
Other comprehensive income (loss), before tax	21	(47)	(477)
Tax effect
Net unrealized gain (loss)	(7)	16	178
Reclassification of net unrealized loss on securities transferred to held to maturity	(85)	0	0
Other reclassifications included in other revenue	4	2	0
Change in net unrealized gain (loss) on held to maturity securities:
Reclassification of net unrealized loss on securities transferred from available for sale (1)	85	0	0
Amortization of amounts previously recorded upon transfer from available for sale	(11)	0	0
Other	4	0	0
Other comprehensive income (loss)	(10)	18	178
Net of tax
Net unrealized gain (loss)	6	(28)	(299)
Reclassification of net unrealized loss on securities transferred to held to maturity	142	0	0
Other reclassifications included in other revenue	(8)	(2)	0
Change in net unrealized gain (loss) on held to maturity securities:
Reclassification of net unrealized loss on securities transferred from available for sale	(142)	0	0
Amortization of amounts previously recorded upon transfer from available for sale	20	0	0
Other	(7)	1	0
Other comprehensive income (loss), net of tax	$ 11	$ (29)	$ (299)