Investments in Securities Maturity Information (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	$ 2,539	$ 3,079
Total fair value	2,761	3,429
One Year or Less, Amortized Cost	2
One Year or Less, Fair Value	2
After One Year through Five Years, Amortized Cost	61
After One Year through Five Years, Fair Value	63
After Five Years through Ten Years, Amortized Cost	199
After Five Years through Ten Years, Fair Value	217
After Ten Years, Amortized Cost	2,277
After Ten Years, Fair Value	2,479
Fannie Mae [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	1,536	1,754
Total fair value	1,615	1,797
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year through Five Years, Amortized Cost	15
After One Year through Five Years, Fair Value	15
After Five Years through Ten Years, Amortized Cost	104
After Five Years through Ten Years, Fair Value	115
After Ten Years, Amortized Cost	1,417
After Ten Years, Fair Value	1,485
Other agency [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	209	239
Total fair value	228	256
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year through Five Years, Amortized Cost	22
After One Year through Five Years, Fair Value	24
After Five Years through Ten Years, Amortized Cost	25
After Five Years through Ten Years, Fair Value	28
After Ten Years, Amortized Cost	162
After Ten Years, Fair Value	176
Alt-A and subprime private-label securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	109	325
Total fair value	220	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	108
After Ten Years, Fair Value	219
Mortgage revenue bonds [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	356	425
Total fair value	364	434
One Year or Less, Amortized Cost	2
One Year or Less, Fair Value	2
After One Year through Five Years, Amortized Cost	24
After One Year through Five Years, Fair Value	24
After Five Years through Ten Years, Amortized Cost	43
After Five Years through Ten Years, Fair Value	45
After Ten Years, Amortized Cost	287
After Ten Years, Fair Value	293
Other mortgage-related securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	329	336
Total fair value	334	$ 350
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	26
After Five Years through Ten Years, Fair Value	28
After Ten Years, Amortized Cost	303
After Ten Years, Fair Value	$ 306