Stockholders' Equity - Adjustments to Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Class of Stock [Line Items]
Net interest revenue	$ 535	$ 561	$ 1,081	$ 1,116
Total reclassifications out of AOCI	393	602	770	958
Reclassification out of Accumulated Other Comprehensive Income
Class of Stock [Line Items]
Total reclassifications out of AOCI	8	13	13	10
Net Unrealized Gains (Losses) on Cash Flow Hedges | Interest-rate contracts | Reclassification out of Accumulated Other Comprehensive Income
Class of Stock [Line Items]
Net interest revenue			1	1
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Tax	0	0	0	1
Net Unrealized Gains (Losses) on Available-for-Sale Securities | Reclassification out of Accumulated Other Comprehensive Income
Class of Stock [Line Items]
Net gains (losses) from sales of available-for-sale securities	(2)	0	(2)	(9)
Gain (Loss) on Investments, Excluding OTTI, Tax	0	0	0	(6)
Other-Than-Temporary Impairment on Held-to-Maturity Securities | Reclassification out of Accumulated Other Comprehensive Income
Class of Stock [Line Items]
Other-than-temporary impairment on held-to-maturity securities related to factors other than credit	0	1	(1)	1
Net Unrealized Losses on Retirement Plans | Reclassification out of Accumulated Other Comprehensive Income
Class of Stock [Line Items]
Amortization of actuarial losses, net of related tax benefits of $3 and $3, respectively	10	12	15	17
OCI reclassification adjustment, pension and other postretirement benefit plans, tax	$ 3	$ (8)	$ (1)	$ (5)