Investments in Securities Maturity Information (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	$ 2,448	$ 3,079
Total fair value	2,690	3,429
One Year or Less, Amortized Cost	2
One Year or Less, Fair Value	2
After One Year through Five Years, Amortized Cost	62
After One Year through Five Years, Fair Value	64
After Five Years through Ten Years, Amortized Cost	188
After Five Years through Ten Years, Fair Value	205
After Ten Years, Amortized Cost	2,196
After Ten Years, Fair Value	2,419
Fannie Mae [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	1,492	1,754
Total fair value	1,582	1,797
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year through Five Years, Amortized Cost	14
After One Year through Five Years, Fair Value	15
After Five Years through Ten Years, Amortized Cost	101
After Five Years through Ten Years, Fair Value	111
After Ten Years, Amortized Cost	1,377
After Ten Years, Fair Value	1,456
Other agency [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	195	239
Total fair value	213	256
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year through Five Years, Amortized Cost	20
After One Year through Five Years, Fair Value	21
After Five Years through Ten Years, Amortized Cost	26
After Five Years through Ten Years, Fair Value	29
After Ten Years, Amortized Cost	149
After Ten Years, Fair Value	163
Alt-A and subprime private-label securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	102	325
Total fair value	226	592
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year through Five Years, Amortized Cost	0
After One Year through Five Years, Fair Value	0
After Five Years through Ten Years, Amortized Cost	1
After Five Years through Ten Years, Fair Value	1
After Ten Years, Amortized Cost	101
After Ten Years, Fair Value	225
Mortgage revenue bonds [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	339	425
Total fair value	346	434
One Year or Less, Amortized Cost	2
One Year or Less, Fair Value	2
After One Year through Five Years, Amortized Cost	26
After One Year through Five Years, Fair Value	27
After Five Years through Ten Years, Amortized Cost	36
After Five Years through Ten Years, Fair Value	37
After Ten Years, Amortized Cost	275
After Ten Years, Fair Value	280
Other mortgage-related securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	320	336
Total fair value	323	$ 350
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year through Five Years, Amortized Cost	2
After One Year through Five Years, Fair Value	1
After Five Years through Ten Years, Amortized Cost	24
After Five Years through Ten Years, Fair Value	27
After Ten Years, Amortized Cost	294
After Ten Years, Fair Value	$ 295