Investments in Securities Maturity Information (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	$ 11,511	$ 18,141
Total fair value	13,180	20,230
One Year or Less, Amortized Cost	533
One Year or Less, Fair Value	540
After One Year through Five Years, Amortized Cost	718
After One Year through Five Years, Fair Value	731
After Five Years through Ten Years, Amortized Cost	465
After Five Years through Ten Years, Fair Value	492
After Ten Years, Amortized Cost	9,795
After Ten Years, Fair Value	11,417
Fannie Mae [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	3,163	4,008
Total fair value	3,336	4,221
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year through Five Years, Amortized Cost	77
After One Year through Five Years, Fair Value	78
After Five Years through Ten Years, Amortized Cost	84
After Five Years through Ten Years, Fair Value	90
After Ten Years, Amortized Cost	3,002
After Ten Years, Fair Value	3,168
Freddie Mac [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	1,899	4,000
Total fair value	2,038	4,299
One Year or Less, Amortized Cost	1
One Year or Less, Fair Value	1
After One Year through Five Years, Amortized Cost	101
After One Year through Five Years, Fair Value	104
After Five Years through Ten Years, Amortized Cost	203
After Five Years through Ten Years, Fair Value	221
After Ten Years, Amortized Cost	1,594
After Ten Years, Fair Value	1,712
Ginnie Mae [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	58	343
Total fair value	66	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	4
After Five Years through Ten Years, Fair Value	4
After Ten Years, Amortized Cost	53
After Ten Years, Fair Value	61
Alt-A private-label securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	1,269	2,029
Total fair value	1,827	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	1,269
After Ten Years, Fair Value	1,827
Subprime private-label securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	1,722	2,526
Total fair value	2,373	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	1,722
After Ten Years, Fair Value	2,373
CMBS [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	1,010	1,235
Total fair value	1,023	1,255
One Year or Less, Amortized Cost	521
One Year or Less, Fair Value	528
After One Year through Five Years, Amortized Cost	444
After One Year through Five Years, Fair Value	451
After Five Years through Ten Years, Amortized Cost	0
After Five Years through Ten Years, Fair Value	0
After Ten Years, Amortized Cost	45
After Ten Years, Fair Value	44
Mortgage revenue bonds [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	1,919	2,639
Total fair value	2,029	2,701
One Year or Less, Amortized Cost	11
One Year or Less, Fair Value	11
After One Year through Five Years, Amortized Cost	95
After One Year through Five Years, Fair Value	96
After Five Years through Ten Years, Amortized Cost	171
After Five Years through Ten Years, Fair Value	174
After Ten Years, Amortized Cost	1,642
After Ten Years, Fair Value	1,748
Other mortgage-related securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	471	1,361
Total fair value	488	$ 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	468
After Ten Years, Fair Value	$ 484