Investments in Securities Maturity Information (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	$ 2,147	$ 2,281
Total fair value	2,289	2,404
Amortized cost of investments in securities	2,147	2,281
Debt Securities, Available-for-sale	2,289	2,404
Estimated of Fair Value [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total fair value	2,289
One Year or Less, Fair Value	2
After One Year through Five Years, Fair Value	63
After Five Years through Ten Years, Fair Value	189
After Ten Years, Fair Value	2,035
Debt Securities, Available-for-sale	2,289
Carrying Value [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total fair value	2,144
One Year or Less, Fair Value	2
After One Year through Five Years, Fair Value	60
After Five Years through Ten Years, Fair Value	171
After Ten Years, Fair Value	1,911
Debt Securities, Available-for-sale	2,144
Fannie Mae [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	1,393	1,445
Total fair value	1,505	1,520
Amortized cost of investments in securities	1,393	1,445
Debt Securities, Available-for-sale	1,505	1,520
Fannie Mae [Member] | Estimated of Fair Value [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total fair value	1,505
One Year or Less, Fair Value	0
After One Year through Five Years, Fair Value	14
After Five Years through Ten Years, Fair Value	109
After Ten Years, Fair Value	1,382
Debt Securities, Available-for-sale	1,505
Fannie Mae [Member] | Carrying Value [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total fair value	1,393
One Year or Less, Fair Value	0
After One Year through Five Years, Fair Value	14
After Five Years through Ten Years, Fair Value	96
After Ten Years, Fair Value	1,283
Debt Securities, Available-for-sale	1,393
Other agency [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	171	183
Total fair value	190	198
Amortized cost of investments in securities	171	183
Debt Securities, Available-for-sale	190	198
Other agency [Member] | Estimated of Fair Value [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total fair value	190
One Year or Less, Fair Value	0
After One Year through Five Years, Fair Value	17
After Five Years through Ten Years, Fair Value	25
After Ten Years, Fair Value	148
Debt Securities, Available-for-sale	190
Other agency [Member] | Carrying Value [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total fair value	171
One Year or Less, Fair Value	0
After One Year through Five Years, Fair Value	16
After Five Years through Ten Years, Fair Value	23
After Ten Years, Fair Value	132
Debt Securities, Available-for-sale	171
Alt-A and subprime private-label securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	4	34
Total fair value	7	57
Amortized cost of investments in securities	4	34
Debt Securities, Available-for-sale	7	57
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	7
One Year or Less, Fair Value	0
After One Year through Five Years, Fair Value	0
After Five Years through Ten Years, Fair Value	3
After Ten Years, Fair Value	4
Debt Securities, Available-for-sale	7
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	4
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	2
Debt Securities, Available-for-sale	4
Mortgage revenue bonds [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	291	309
Total fair value	296	315
Amortized cost of investments in securities	291	309
Debt Securities, Available-for-sale	296	315
Mortgage revenue bonds [Member] | Estimated of Fair Value [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total fair value	296
One Year or Less, Fair Value	2
After One Year through Five Years, Fair Value	32
After Five Years through Ten Years, Fair Value	28
After Ten Years, Fair Value	234
Debt Securities, Available-for-sale	296
Mortgage revenue bonds [Member] | Carrying Value [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total fair value	288
One Year or Less, Fair Value	2
After One Year through Five Years, Fair Value	30
After Five Years through Ten Years, Fair Value	27
After Ten Years, Fair Value	229
Debt Securities, Available-for-sale	288
Other mortgage-related securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	288	310
Total fair value	291	314
Amortized cost of investments in securities	288	310
Debt Securities, Available-for-sale	291	$ 314
Other mortgage-related securities [Member] | Estimated of Fair Value [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total fair value	291
One Year or Less, Fair Value	0
After One Year through Five Years, Fair Value	0
After Five Years through Ten Years, Fair Value	24
After Ten Years, Fair Value	267
Debt Securities, Available-for-sale	291
Other mortgage-related securities [Member] | Carrying Value [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total fair value	288
One Year or Less, Fair Value	0
After One Year through Five Years, Fair Value	0
After Five Years through Ten Years, Fair Value	23
After Ten Years, Fair Value	265
Debt Securities, Available-for-sale	$ 288