Investments in Securities - Maturity Information (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost and fair value of AFS	$ 955	$ 1,697
Fair Value
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost and fair value of AFS	955
One Year or Less	1
After One Year Through Five Years	27
After Five Years Through Ten Years	41
After Ten Years	886
Carrying Amount
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost and fair value of AFS	929
One Year or Less	1
After One Year Through Five Years	26
After Five Years Through Ten Years	38
After Ten Years	864
Fannie Mae
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost and fair value of AFS	563	1,168
Fannie Mae | Fair Value
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost and fair value of AFS	563
One Year or Less	0
After One Year Through Five Years	3
After Five Years Through Ten Years	14
After Ten Years	546
Fannie Mae | Carrying Amount
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost and fair value of AFS	550
One Year or Less	0
After One Year Through Five Years	3
After Five Years Through Ten Years	12
After Ten Years	535
Other agency
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost and fair value of AFS	15	65
Other agency | Fair Value
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost and fair value of AFS	15
One Year or Less	0
After One Year Through Five Years	0
After Five Years Through Ten Years	1
After Ten Years	14
Other agency | Carrying Amount
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost and fair value of AFS	14
One Year or Less	0
After One Year Through Five Years	0
After Five Years Through Ten Years	1
After Ten Years	13
Alt-A and subprime private-label securities
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost and fair value of AFS	5	6
Alt-A and subprime private-label securities | Fair Value
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost and fair value of AFS	5
One Year or Less	0
After One Year Through Five Years	0
After Five Years Through Ten Years	2
After Ten Years	3
Alt-A and subprime private-label securities | Carrying Amount
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost and fair value of AFS	4
One Year or Less	0
After One Year Through Five Years	0
After Five Years Through Ten Years	2
After Ten Years	2
Mortgage revenue bonds
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost and fair value of AFS	172	216
Mortgage revenue bonds | Fair Value
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost and fair value of AFS	172
One Year or Less	1
After One Year Through Five Years	24
After Five Years Through Ten Years	19
After Ten Years	128
Mortgage revenue bonds | Carrying Amount
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost and fair value of AFS	166
One Year or Less	1
After One Year Through Five Years	23
After Five Years Through Ten Years	19
After Ten Years	123
Other mortgage-related securities
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost and fair value of AFS	200	$ 242
Other mortgage-related securities | Fair Value
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost and fair value of AFS	200
One Year or Less	0
After One Year Through Five Years	0
After Five Years Through Ten Years	5
After Ten Years	195
Other mortgage-related securities | Carrying Amount
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost and fair value of AFS	195
One Year or Less	0
After One Year Through Five Years	0
After Five Years Through Ten Years	4
After Ten Years	$ 191