Investments in Securities Maturity Information (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	$ 8,512	$ 18,141
Total fair value	9,865	20,230
One Year or Less, Amortized Cost	787
One Year or Less, Fair Value	793
After One Year through Five Years, Amortized Cost	337
After One Year through Five Years, Fair Value	343
After Five Years through Ten Years, Amortized Cost	331
After Five Years through Ten Years, Fair Value	347
After Ten Years, Amortized Cost	7,057
After Ten Years, Fair Value	8,382
Fannie Mae [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	2,545	4,008
Total fair value	2,712	4,221
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year through Five Years, Amortized Cost	29
After One Year through Five Years, Fair Value	29
After Five Years through Ten Years, Amortized Cost	60
After Five Years through Ten Years, Fair Value	66
After Ten Years, Amortized Cost	2,456
After Ten Years, Fair Value	2,617
Freddie Mac [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	534	4,000
Total fair value	582	4,299
One Year or Less, Amortized Cost	1
One Year or Less, Fair Value	1
After One Year through Five Years, Amortized Cost	65
After One Year through Five Years, Fair Value	68
After Five Years through Ten Years, Amortized Cost	95
After Five Years through Ten Years, Fair Value	102
After Ten Years, Amortized Cost	373
After Ten Years, Fair Value	411
Ginnie Mae [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	54	343
Total fair value	61	391
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year through Five Years, Amortized Cost	1
After One Year through Five Years, Fair Value	1
After Five Years through Ten Years, Amortized Cost	3
After Five Years through Ten Years, Fair Value	4
After Ten Years, Amortized Cost	50
After Ten Years, Fair Value	56
Alt-A private-label securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	883	2,029
Total fair value	1,417	2,678
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	883
After Ten Years, Fair Value	1,417
Subprime private-label securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	1,437	2,526
Total fair value	1,919	3,281
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	1,437
After Ten Years, Fair Value	1,919
CMBS [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	971	1,235
Total fair value	978	1,255
One Year or Less, Amortized Cost	776
One Year or Less, Fair Value	782
After One Year through Five Years, Amortized Cost	154
After One Year through Five Years, Fair Value	156
After Five Years through Ten Years, Amortized Cost	0
After Five Years through Ten Years, Fair Value	0
After Ten Years, Amortized Cost	41
After Ten Years, Fair Value	40
Mortgage revenue bonds [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	1,639	2,639
Total fair value	1,725	2,701
One Year or Less, Amortized Cost	10
One Year or Less, Fair Value	10
After One Year through Five Years, Amortized Cost	88
After One Year through Five Years, Fair Value	88
After Five Years through Ten Years, Amortized Cost	170
After Five Years through Ten Years, Fair Value	172
After Ten Years, Amortized Cost	1,371
After Ten Years, Fair Value	1,455
Other mortgage-related securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	449	1,361
Total fair value	471	$ 1,404
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	1
After Five Years through Ten Years, Amortized Cost	3
After Five Years through Ten Years, Fair Value	3
After Ten Years, Amortized Cost	446
After Ten Years, Fair Value	$ 467