Investments in Securities Maturity Information (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total Amortized Cost	$ 19,587	$ 27,413
Total Available-for-Sale, at Fair Value	22,007	30,654
One Year or Less, Amortized Cost	183
One Year or Less, Fair Value	185
After One Year through Five Years, Amortized Cost	1,554
After One Year through Five Years, Fair Value	1,605
After Five Years through Ten Years, Amortized Cost	804
After Five Years through Ten Years, Fair Value	859
After Ten Years, Amortized Cost	17,046
After Ten Years, Fair Value	19,358
Fannie Mae [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total Amortized Cost	4,026	5,330
Total Available-for-Sale, at Fair Value	4,282	5,639
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year through Five Years, Amortized Cost	189
After One Year through Five Years, Fair Value	194
After Five Years through Ten Years, Amortized Cost	165
After Five Years through Ten Years, Fair Value	178
After Ten Years, Amortized Cost	3,672
After Ten Years, Fair Value	3,910
Freddie Mac [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total Amortized Cost	4,252	5,100
Total Available-for-Sale, at Fair Value	4,599	5,528
One Year or Less, Amortized Cost	1
One Year or Less, Fair Value	1
After One Year through Five Years, Amortized Cost	221
After One Year through Five Years, Fair Value	231
After Five Years through Ten Years, Amortized Cost	332
After Five Years through Ten Years, Fair Value	363
After Ten Years, Amortized Cost	3,698
After Ten Years, Fair Value	4,004
Ginnie Mae [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total Amortized Cost	361	416
Total Available-for-Sale, at Fair Value	413	476
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	57
After Five Years through Ten Years, Fair Value	64
After Ten Years, Amortized Cost	303
After Ten Years, Fair Value	348
Alt-A private-label securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total Amortized Cost	2,626	4,638
Total Available-for-Sale, at Fair Value	3,349	5,678
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	2,626
After Ten Years, Fair Value	3,349
Subprime private-label securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total Amortized Cost	2,817	4,103
Total Available-for-Sale, at Fair Value	3,677	5,240
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	2,817
After Ten Years, Fair Value	3,677
CMBS [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total Amortized Cost	1,273	1,341
Total Available-for-Sale, at Fair Value	1,309	1,397
One Year or Less, Amortized Cost	171
One Year or Less, Fair Value	173
After One Year through Five Years, Amortized Cost	1,037
After One Year through Five Years, Fair Value	1,071
After Five Years through Ten Years, Amortized Cost	0
After Five Years through Ten Years, Fair Value	0
After Ten Years, Amortized Cost	65
After Ten Years, Fair Value	65
Mortgage revenue bonds [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total Amortized Cost	2,824	3,859
Total Available-for-Sale, at Fair Value	2,933	4,023
One Year or Less, Amortized Cost	11
One Year or Less, Fair Value	11
After One Year through Five Years, Amortized Cost	106
After One Year through Five Years, Fair Value	108
After Five Years through Ten Years, Amortized Cost	219
After Five Years through Ten Years, Fair Value	221
After Ten Years, Amortized Cost	2,488
After Ten Years, Fair Value	2,593
Other mortgage-related securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total Amortized Cost	1,408	2,626
Total Available-for-Sale, at Fair Value	1,445	$ 2,673
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	31
After Five Years through Ten Years, Fair Value	33
After Ten Years, Amortized Cost	1,377
After Ten Years, Fair Value	$ 1,412