Other-Than-Temporary Impairment Analysis (OTTI Impaired PLRMBS) (Details) - Subtotal PLRMBS - Residential Mortgage Backed Securities - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Available-for-sale Securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Unpaid Principal Balance	$ 2,892	$ 3,548
Amortized Cost	2,329	2,870
Estimated Fair Value	2,597	3,157
Held-to-maturity Securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Unpaid Principal Balance	39	52
Amortized Cost	35	47
Estimated Fair Value	39	51
Carrying Value	33	44
Prime | Available-for-sale Securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Unpaid Principal Balance	258	323
Amortized Cost	213	267
Estimated Fair Value	230	288
Prime | Held-to-maturity Securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Unpaid Principal Balance	0	0
Amortized Cost	0	0
Estimated Fair Value	0	0
Carrying Value	0	0
Alt-A | Available-for-sale Securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Unpaid Principal Balance	2,634	3,225
Amortized Cost	2,116	2,603
Estimated Fair Value	2,367	2,869
Alt-A | Held-to-maturity Securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Unpaid Principal Balance	39	52
Amortized Cost	35	47
Estimated Fair Value	39	51
Carrying Value	$ 33	$ 44