Investments in Securities Maturity Information (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	$ 5,419	$ 7,261
Total fair value	6,408	8,363
One Year or Less, Amortized Cost	272
One Year or Less, Fair Value	272
After One Year through Five Years, Amortized Cost	114
After One Year through Five Years, Fair Value	115
After Five Years through Ten Years, Amortized Cost	241
After Five Years through Ten Years, Fair Value	251
After Ten Years, Amortized Cost	4,792
After Ten Years, Fair Value	5,770
Fannie Mae [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	2,173	2,445
Total fair value	2,272	2,554
One Year or Less, Amortized Cost	2
One Year or Less, Fair Value	2
After One Year through Five Years, Amortized Cost	13
After One Year through Five Years, Fair Value	14
After Five Years through Ten Years, Amortized Cost	64
After Five Years through Ten Years, Fair Value	68
After Ten Years, Amortized Cost	2,094
After Ten Years, Fair Value	2,188
Other agency [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	399	508
Total fair value	432	547
One Year or Less, Amortized Cost	1
One Year or Less, Fair Value	1
After One Year through Five Years, Amortized Cost	26
After One Year through Five Years, Fair Value	26
After Five Years through Ten Years, Amortized Cost	62
After Five Years through Ten Years, Fair Value	66
After Ten Years, Amortized Cost	310
After Ten Years, Fair Value	339
Alt-A and subprime private-label securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	1,335	1,817
Total fair value	2,162	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	1,335
After Ten Years, Fair Value	2,162
CMBS [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	259	815
Total fair value	259	819
One Year or Less, Amortized Cost	259
One Year or Less, Fair Value	259
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	858	1,245
Total fair value	873	1,272
One Year or Less, Amortized Cost	10
One Year or Less, Fair Value	10
After One Year through Five Years, Amortized Cost	75
After One Year through Five Years, Fair Value	75
After Five Years through Ten Years, Amortized Cost	112
After Five Years through Ten Years, Fair Value	114
After Ten Years, Amortized Cost	661
After Ten Years, Fair Value	674
Other mortgage-related securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	395	431
Total fair value	410	$ 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	3
After Five Years through Ten Years, Fair Value	3
After Ten Years, Amortized Cost	392
After Ten Years, Fair Value	$ 407