Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' Equity Attributable to Parent	$ 6,407	$ 6,646	$ 6,741	$ 6,530
Accumulated Other-than-Temporary Impairment | Available-for-sale Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' Equity Attributable to Parent	0	301	268	287
Net change in fair value	0	14
Reclassification from other comprehensive income/(loss) to net income/(loss) [Abstract]
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	0	14
Accumulated Other-than-Temporary Impairment | Held-to-maturity Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' Equity Attributable to Parent	(1)	(3)	(1)	(3)
Reclassification from other comprehensive income/(loss) to net income/(loss) [Abstract]
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	0	0
Accumulated Defined Benefit Plans Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' Equity Attributable to Parent	(14)	(17)	(14)	(17)
Reclassification from other comprehensive income/(loss) to net income/(loss) [Abstract]
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	0	0
Accumulated Other Comprehensive Income/(Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' Equity Attributable to Parent	(228)	287	274	235
Net change in fair value	(502)	52
Reclassification from other comprehensive income/(loss) to net income/(loss) [Abstract]
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(502)	52
Available-for-sale Securities | AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' Equity Attributable to Parent	(213)	6	21	$ (32)
AFS Unrealized Gain/(Loss) on AFS Securities	(502)	38
Reclassification from other comprehensive income/(loss) to net income/(loss) [Abstract]
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	$ (502)	$ 38
Cumulative Effect, Period of Adoption, Adjustment [Member] | Accounting Standards Update 2016-13 [Member] | Accumulated Other-than-Temporary Impairment | Available-for-sale Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' Equity Attributable to Parent			(268)
Cumulative Effect, Period of Adoption, Adjustment [Member] | Accounting Standards Update 2016-13 [Member] | Accumulated Other Comprehensive Income/(Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' Equity Attributable to Parent			0
Cumulative Effect, Period of Adoption, Adjustment [Member] | Accounting Standards Update 2016-13 [Member] | AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' Equity Attributable to Parent			$ 268