Investments in Securities Available-for-sale Securities 2 (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	$ 1,440	$ 1,606
Allowance for credit losses	(3)
Gross unrealized gains	76	106
Gross unrealized losses	(11)	(12)
Total fair value	1,502	1,697
Interest excluded from amortized cost and fair value	5	6
Fannie Mae [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	988	1,094
Allowance for credit losses	0	0
Gross unrealized gains	59	86
Gross unrealized losses	(11)	(12)
Total fair value	1,036	1,168
Other agency [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	32	59
Allowance for credit losses	0	0
Gross unrealized gains	2	6
Gross unrealized losses	0	0
Total fair value	34	65
Alt-A and subprime private-label securities [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	3	4
Allowance for credit losses	0	0
Gross unrealized gains	2	2
Gross unrealized losses	0	0
Total fair value	5	6
Mortgage revenue bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	187	211
Allowance for credit losses	(3)
Gross unrealized gains	7	8
Gross unrealized losses	0	0
Total fair value	191	216
Other mortgage-related securities [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	230	238
Allowance for credit losses	0
Gross unrealized gains	6	4
Gross unrealized losses	0	0
Total fair value	$ 236	$ 242