Investments in Securities Maturity Information (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	$ 1,440	$ 1,606
Total fair value	1,502	1,697
Available-for-sale, at fair value (with an amortized cost of $1,440 and $1,606, net of allowance of credit losses of $3)	1,440	1,606
Total fair value	1,502	1,697
Estimated of Fair Value [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total fair value	1,502
One Year or Less, Fair Value	1
After One Year through Five Years, Fair Value	31
After Five Years through Ten Years, Fair Value	151
After Ten Years, Fair Value	1,319
Total fair value	1,502
Carrying Value [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total fair value	1,437
One Year or Less, Fair Value	1
After One Year through Five Years, Fair Value	30
After Five Years through Ten Years, Fair Value	137
After Ten Years, Fair Value	1,269
Total fair value	1,437
Fannie Mae [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	988	1,094
Total fair value	1,036	1,168
Available-for-sale, at fair value (with an amortized cost of $1,440 and $1,606, net of allowance of credit losses of $3)	988	1,094
Total fair value	1,036	1,168
Fannie Mae [Member] | Estimated of Fair Value [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total fair value	1,036
One Year or Less, Fair Value	0
After One Year through Five Years, Fair Value	4
After Five Years through Ten Years, Fair Value	109
After Ten Years, Fair Value	923
Total fair value	1,036
Fannie Mae [Member] | Carrying Value [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total fair value	988
One Year or Less, Fair Value	0
After One Year through Five Years, Fair Value	4
After Five Years through Ten Years, Fair Value	98
After Ten Years, Fair Value	886
Total fair value	988
Other agency [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	32	59
Total fair value	34	65
Available-for-sale, at fair value (with an amortized cost of $1,440 and $1,606, net of allowance of credit losses of $3)	32	59
Total fair value	34	65
Other agency [Member] | Estimated of Fair Value [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total fair value	34
One Year or Less, Fair Value	0
After One Year through Five Years, Fair Value	0
After Five Years through Ten Years, Fair Value	1
After Ten Years, Fair Value	33
Total fair value	34
Other agency [Member] | Carrying Value [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total fair value	32
One Year or Less, Fair Value	0
After One Year through Five Years, Fair Value	0
After Five Years through Ten Years, Fair Value	1
After Ten Years, Fair Value	31
Total fair value	32
Alt-A and subprime private-label securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	3	4
Total fair value	5	6
Available-for-sale, at fair value (with an amortized cost of $1,440 and $1,606, net of allowance of credit losses of $3)	3	4
Total fair value	5	6
Alt-A and subprime private-label securities [Member] | Estimated of Fair Value [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total fair value	5
One Year or Less, Fair Value	0
After One Year through Five Years, Fair Value	0
After Five Years through Ten Years, Fair Value	2
After Ten Years, Fair Value	3
Total fair value	5
Alt-A and subprime private-label securities [Member] | Carrying Value [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total fair value	3
One Year or Less, Fair Value	0
After One Year through Five Years, Fair Value	0
After Five Years through Ten Years, Fair Value	2
After Ten Years, Fair Value	1
Total fair value	3
Mortgage revenue bonds [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	187	211
Total fair value	191	216
Available-for-sale, at fair value (with an amortized cost of $1,440 and $1,606, net of allowance of credit losses of $3)	187	211
Total fair value	191	216
Mortgage revenue bonds [Member] | Estimated of Fair Value [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total fair value	191
One Year or Less, Fair Value	1
After One Year through Five Years, Fair Value	27
After Five Years through Ten Years, Fair Value	19
After Ten Years, Fair Value	144
Total fair value	191
Mortgage revenue bonds [Member] | Carrying Value [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total fair value	184
One Year or Less, Fair Value	1
After One Year through Five Years, Fair Value	26
After Five Years through Ten Years, Fair Value	18
After Ten Years, Fair Value	139
Total fair value	184
Other mortgage-related securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	230	238
Total fair value	236	242
Available-for-sale, at fair value (with an amortized cost of $1,440 and $1,606, net of allowance of credit losses of $3)	230	238
Total fair value	236	$ 242
Other mortgage-related securities [Member] | Estimated of Fair Value [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total fair value	236
One Year or Less, Fair Value	0
After One Year through Five Years, Fair Value	0
After Five Years through Ten Years, Fair Value	20
After Ten Years, Fair Value	216
Total fair value	236
Other mortgage-related securities [Member] | Carrying Value [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total fair value	230
One Year or Less, Fair Value	0
After One Year through Five Years, Fair Value	0
After Five Years through Ten Years, Fair Value	18
After Ten Years, Fair Value	212
Total fair value	$ 230