Investments in Securities - Available-for-sale Securities 1 (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	$ 932	$ 1,606
Allowance for Credit Losses	(3)	(3)
Gross Unrealized Gains in AOCI	33	106
Gross Unrealized Losses in AOCI	(7)	(12)
Available-for-sale, at fair value	955	1,697
Interest excluded from amortized cost and fair value	3	6
Fannie Mae
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	550	1,094
Allowance for Credit Losses	0	0
Gross Unrealized Gains in AOCI	20	86
Gross Unrealized Losses in AOCI	(7)	(12)
Available-for-sale, at fair value	563	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	4	4
Allowance for Credit Losses	0	0
Gross Unrealized Gains in AOCI	1	2
Gross Unrealized Losses in AOCI	0	0
Available-for-sale, at fair value	5	6
Mortgage revenue bonds
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	169	211
Allowance for Credit Losses	(3)	(3)
Gross Unrealized Gains in AOCI	6	8
Gross Unrealized Losses in AOCI	0	0
Available-for-sale, at fair value	172	216
Other mortgage-related securities
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	195	238
Allowance for Credit Losses	0	0
Gross Unrealized Gains in AOCI	5	4
Gross Unrealized Losses in AOCI	0	0
Available-for-sale, at fair value	$ 200	$ 242