Investments in Securities Available-for-sale Securities 2 (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	$ 1,606	$ 2,281
Allowance for Credit Losses	(3)
Gross unrealized gains	106	137
Gross unrealized losses	(12)	(14)
Total fair value	1,697	2,404
Debt Securities, Available-for-Sale, Accrued Interest, after Allowance for Credit Loss	6	10
Fannie Mae [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	1,094	1,445
Allowance for Credit Losses	0
Gross unrealized gains	86	85
Gross unrealized losses	(12)	(10)
Total fair value	1,168	1,520
Other agency [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	59	183
Allowance for Credit Losses	0
Gross unrealized gains	6	15
Gross unrealized losses	0	0
Total fair value	65	198
Alt-A and subprime private-label securities [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	4	34
Allowance for Credit Losses	0
Gross unrealized gains	2	23
Gross unrealized losses	0	0
Total fair value	6	57
Mortgage revenue bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	211	309
Allowance for Credit Losses	(3)
Gross unrealized gains	8	9
Gross unrealized losses	0	(3)
Total fair value	216	315
Other mortgage-related securities [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	238	310
Allowance for Credit Losses	0
Gross unrealized gains	4	5
Gross unrealized losses	0	(1)
Total fair value	$ 242	$ 314