Investments in Securities Maturity Information (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total Amortized Cost	$ 21,491	$ 27,413
Total Available-for-Sale, at Fair Value	24,161	30,654
One Year or Less, Amortized Cost	201
One Year or Less, Fair Value	204
After One Year through Five Years, Amortized Cost	1,633
After One Year through Five Years, Fair Value	1,694
After Five Years through Ten Years, Amortized Cost	899
After Five Years through Ten Years, Fair Value	961
After Ten Years, Amortized Cost	18,758
After Ten Years, Fair Value	21,302
Fannie Mae [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total Amortized Cost	4,376	5,330
Total Available-for-Sale, at Fair Value	4,638	5,639
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year through Five Years, Amortized Cost	220
After One Year through Five Years, Fair Value	228
After Five Years through Ten Years, Amortized Cost	190
After Five Years through Ten Years, Fair Value	205
After Ten Years, Amortized Cost	3,966
After Ten Years, Fair Value	4,205
Freddie Mac [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total Amortized Cost	4,565	5,100
Total Available-for-Sale, at Fair Value	4,947	5,528
One Year or Less, Amortized Cost	1
One Year or Less, Fair Value	1
After One Year through Five Years, Amortized Cost	269
After One Year through Five Years, Fair Value	282
After Five Years through Ten Years, Amortized Cost	370
After Five Years through Ten Years, Fair Value	404
After Ten Years, Amortized Cost	3,925
After Ten Years, Fair Value	4,260
Ginnie Mae [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total Amortized Cost	380	416
Total Available-for-Sale, at Fair Value	435	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	56
After Five Years through Ten Years, Fair Value	63
After Ten Years, Amortized Cost	323
After Ten Years, Fair Value	371
Alt-A private-label securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total Amortized Cost	2,877	4,638
Total Available-for-Sale, at Fair Value	3,630	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	2,877
After Ten Years, Fair Value	3,630
Subprime private-label securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total Amortized Cost	2,932	4,103
Total Available-for-Sale, at Fair Value	3,837	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	2,932
After Ten Years, Fair Value	3,837
CMBS [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total Amortized Cost	1,301	1,341
Total Available-for-Sale, at Fair Value	1,344	1,397
One Year or Less, Amortized Cost	187
One Year or Less, Fair Value	190
After One Year through Five Years, Amortized Cost	1,043
After One Year through Five Years, Fair Value	1,082
After Five Years through Ten Years, Amortized Cost	0
After Five Years through Ten Years, Fair Value	0
After Ten Years, Amortized Cost	71
After Ten Years, Fair Value	72
Mortgage revenue bonds [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total Amortized Cost	3,032	3,859
Total Available-for-Sale, at Fair Value	3,171	4,023
One Year or Less, Amortized Cost	13
One Year or Less, Fair Value	13
After One Year through Five Years, Amortized Cost	100
After One Year through Five Years, Fair Value	101
After Five Years through Ten Years, Amortized Cost	250
After Five Years through Ten Years, Fair Value	253
After Ten Years, Amortized Cost	2,669
After Ten Years, Fair Value	2,804
Other mortgage-related securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total Amortized Cost	2,028	2,626
Total Available-for-Sale, at Fair Value	2,159	$ 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	33
After Five Years through Ten Years, Fair Value	36
After Ten Years, Amortized Cost	1,995
After Ten Years, Fair Value	$ 2,123