Investments in Securities Maturity Information (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	$ 1,990	$ 2,281
Total fair value	2,120	2,404
Amortized cost of investments in securities	1,990	2,281
Total fair value	2,120	2,404
Estimated of Fair Value [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total fair value	2,120
One Year or Less, Fair Value	2
After One Year through Five Years, Fair Value	61
After Five Years through Ten Years, Fair Value	180
After Ten Years, Fair Value	1,877
Total fair value	2,120
Carrying Value [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total fair value	1,987
One Year or Less, Fair Value	2
After One Year through Five Years, Fair Value	57
After Five Years through Ten Years, Fair Value	163
After Ten Years, Fair Value	1,765
Total fair value	1,987
Fannie Mae [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	1,296	1,445
Total fair value	1,394	1,520
Amortized cost of investments in securities	1,296	1,445
Total fair value	1,394	1,520
Fannie Mae [Member] | Estimated of Fair Value [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total fair value	1,394
One Year or Less, Fair Value	0
After One Year through Five Years, Fair Value	13
After Five Years through Ten Years, Fair Value	105
After Ten Years, Fair Value	1,276
Total fair value	1,394
Fannie Mae [Member] | Carrying Value [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total fair value	1,296
One Year or Less, Fair Value	0
After One Year through Five Years, Fair Value	12
After Five Years through Ten Years, Fair Value	93
After Ten Years, Fair Value	1,191
Total fair value	1,296
Other agency [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	161	183
Total fair value	180	198
Amortized cost of investments in securities	161	183
Total fair value	180	198
Other agency [Member] | Estimated of Fair Value [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total fair value	180
One Year or Less, Fair Value	0
After One Year through Five Years, Fair Value	15
After Five Years through Ten Years, Fair Value	24
After Ten Years, Fair Value	141
Total fair value	180
Other agency [Member] | Carrying Value [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total fair value	161
One Year or Less, Fair Value	0
After One Year through Five Years, Fair Value	14
After Five Years through Ten Years, Fair Value	22
After Ten Years, Fair Value	125
Total fair value	161
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
Total fair value	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
Total fair value	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	3
After Ten Years, Fair Value	1
Total fair value	4
Mortgage revenue bonds [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	272	309
Total fair value	278	315
Amortized cost of investments in securities	272	309
Total fair value	278	315
Mortgage revenue bonds [Member] | Estimated of Fair Value [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total fair value	278
One Year or Less, Fair Value	2
After One Year through Five Years, Fair Value	33
After Five Years through Ten Years, Fair Value	25
After Ten Years, Fair Value	218
Total fair value	278
Mortgage revenue bonds [Member] | Carrying Value [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total fair value	269
One Year or Less, Fair Value	2
After One Year through Five Years, Fair Value	31
After Five Years through Ten Years, Fair Value	24
After Ten Years, Fair Value	212
Total fair value	269
Other mortgage-related securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	257	310
Total fair value	261	314
Amortized cost of investments in securities	257	310
Total fair value	261	$ 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	261
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	238
Total fair value	261
Other mortgage-related securities [Member] | Carrying Value [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total fair value	257
One Year or Less, Fair Value	0
After One Year through Five Years, Fair Value	0
After Five Years through Ten Years, Fair Value	21
After Ten Years, Fair Value	236
Total fair value	$ 257