Investments in Securities - Available-for-sale Securities 1 (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	$ 873	$ 1,606
Allowance for Credit Losses	0	(3)
Gross Unrealized Gains in AOCI	23	106
Gross Unrealized Losses in AOCI	(10)	(12)
Available-for-sale, at fair value	886	1,697
Interest excluded from amortized cost and fair value	3	6
Maximum
Debt Securities, Available-for-sale [Line Items]
Allowance for Credit Losses	(1)
Fannie Mae
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	510	1,094
Allowance for Credit Losses	0	0
Gross Unrealized Gains in AOCI	16	86
Gross Unrealized Losses in AOCI	(8)	(12)
Available-for-sale, at fair value	518	1,168
Other agency
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	14	59
Allowance for Credit Losses	0	0
Gross Unrealized Gains in AOCI	1	6
Gross Unrealized Losses in AOCI	0	0
Available-for-sale, at fair value	15	65
Alt-A and subprime private-label securities
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	3	4
Allowance for Credit Losses	0	0
Gross Unrealized Gains in AOCI	0	2
Gross Unrealized Losses in AOCI	0	0
Available-for-sale, at fair value	3	6
Mortgage revenue bonds
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	161	211
Allowance for Credit Losses	0	(3)
Gross Unrealized Gains in AOCI	5	8
Gross Unrealized Losses in AOCI	(2)	0
Available-for-sale, at fair value	164	216
Other mortgage-related securities
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	185	238
Allowance for Credit Losses	0	0
Gross Unrealized Gains in AOCI	1	4
Gross Unrealized Losses in AOCI	0	0
Available-for-sale, at fair value	$ 186	$ 242