Investments in Securities Maturity Information (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	$ 2,910	$ 3,079
Total fair value	3,214	3,429
One Year or Less, Amortized Cost	2
One Year or Less, Fair Value	2
After One Year through Five Years, Amortized Cost	67
After One Year through Five Years, Fair Value	69
After Five Years through Ten Years, Amortized Cost	224
After Five Years through Ten Years, Fair Value	242
After Ten Years, Amortized Cost	2,617
After Ten Years, Fair Value	2,901
Fannie Mae [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	1,689	1,754
Total fair value	1,752	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	114
After Five Years through Ten Years, Fair Value	124
After Ten Years, Amortized Cost	1,561
After Ten Years, Fair Value	1,613
Other agency [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	225	239
Total fair value	242	256
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year through Five Years, Amortized Cost	23
After One Year through Five Years, Fair Value	24
After Five Years through Ten Years, Amortized Cost	29
After Five Years through Ten Years, Fair Value	32
After Ten Years, Amortized Cost	173
After Ten Years, Fair Value	186
Alt-A and subprime private-label securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	246	325
Total fair value	452	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	245
After Ten Years, Fair Value	451
Mortgage revenue bonds [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	416	425
Total fair value	425	434
One Year or Less, Amortized Cost	2
One Year or Less, Fair Value	2
After One Year through Five Years, Amortized Cost	30
After One Year through Five Years, Fair Value	30
After Five Years through Ten Years, Amortized Cost	52
After Five Years through Ten Years, Fair Value	54
After Ten Years, Amortized Cost	332
After Ten Years, Fair Value	339
Other mortgage-related securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	334	336
Total fair value	343	$ 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	28
After Five Years through Ten Years, Fair Value	31
After Ten Years, Amortized Cost	306
After Ten Years, Fair Value	$ 312