Investments in Securities Available-for-sale Securities 2 (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	$ 1,780	$ 2,281
Allowance for credit losses	(3)
Gross unrealized gains	132	137
Gross unrealized losses	(14)	(14)
Total fair value	1,895	2,404
Interest excluded from amortized cost and fair value	7	10
Fannie Mae [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	1,195	1,445
Allowance for credit losses	0
Gross unrealized gains	102	85
Gross unrealized losses	(14)	(10)
Total fair value	1,283	1,520
Other agency [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	85	183
Allowance for credit losses	0
Gross unrealized gains	11	15
Gross unrealized losses	0	0
Total fair value	96	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	249	309
Allowance for credit losses	(3)
Gross unrealized gains	9	9
Gross unrealized losses	0	(3)
Total fair value	255	315
Other mortgage-related securities [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	247	310
Allowance for credit losses	0
Gross unrealized gains	8	5
Gross unrealized losses	0	(1)
Total fair value	$ 255	$ 314