Investments in Securities Available-for-sale Securities 2 (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	$ 2,147	$ 2,281
Gross unrealized gains	157	137
Gross unrealized losses	(12)	(14)
Total fair value	2,289	2,404
Allowance for credit losses	(3)
Debt Securities, Available-for-Sale, Accrued Interest, after Allowance for Credit Loss	8	10
Fannie Mae [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	1,393	1,445
Gross unrealized gains	123	85
Gross unrealized losses	(11)	(10)
Total fair value	1,505	1,520
Allowance for credit losses	0
Other agency [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	171	183
Gross unrealized gains	19	15
Gross unrealized losses	0	0
Total fair value	190	198
Allowance for credit losses	0
Alt-A and subprime private-label securities [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	4	34
Gross unrealized gains	3	23
Gross unrealized losses	0	0
Total fair value	7	57
Allowance for credit losses	0
Mortgage revenue bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	291	309
Gross unrealized gains	9	9
Gross unrealized losses	(1)	(3)
Total fair value	296	315
Allowance for credit losses	(3)
Other mortgage-related securities [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	288	310
Gross unrealized gains	3	5
Gross unrealized losses	0	(1)
Total fair value	291	$ 314
Allowance for credit losses	$ 0