Investments in Securities Maturity Information (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	$ 4,932	$ 7,261
Total fair value	5,963	8,363
One Year or Less, Amortized Cost	200
One Year or Less, Fair Value	200
After One Year through Five Years, Amortized Cost	99
After One Year through Five Years, Fair Value	100
After Five Years through Ten Years, Amortized Cost	235
After Five Years through Ten Years, Fair Value	246
After Ten Years, Amortized Cost	4,398
After Ten Years, Fair Value	5,417
Fannie Mae [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	2,118	2,445
Total fair value	2,208	2,554
One Year or Less, Amortized Cost	4
One Year or Less, Fair Value	4
After One Year through Five Years, Amortized Cost	11
After One Year through Five Years, Fair Value	11
After Five Years through Ten Years, Amortized Cost	64
After Five Years through Ten Years, Fair Value	68
After Ten Years, Amortized Cost	2,039
After Ten Years, Fair Value	2,125
Other agency [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	361	508
Total fair value	391	547
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	23
After Five Years through Ten Years, Amortized Cost	58
After Five Years through Ten Years, Fair Value	62
After Ten Years, Amortized Cost	281
After Ten Years, Fair Value	306
Alt-A and subprime private-label securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	1,155	1,817
Total fair value	2,034	2,709
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	0
After Five Years through Ten Years, Fair Value	0
After Ten Years, Amortized Cost	1,155
After Ten Years, Fair Value	2,034
CMBS [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	182	815
Total fair value	182	819
One Year or Less, Amortized Cost	182
One Year or Less, Fair Value	182
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	0
After Five Years through Ten Years, Fair Value	0
After Ten Years, Amortized Cost	0
After Ten Years, Fair Value	0
Mortgage revenue bonds [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	736	1,245
Total fair value	749	1,272
One Year or Less, Amortized Cost	14
One Year or Less, Fair Value	14
After One Year through Five Years, Amortized Cost	66
After One Year through Five Years, Fair Value	66
After Five Years through Ten Years, Amortized Cost	103
After Five Years through Ten Years, Fair Value	105
After Ten Years, Amortized Cost	553
After Ten Years, Fair Value	564
Other mortgage-related securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	380	431
Total fair value	399	$ 462
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	10
After Five Years through Ten Years, Fair Value	11
After Ten Years, Amortized Cost	370
After Ten Years, Fair Value	$ 388