Investments in Securities Maturity Information (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total Amortized Cost	$ 24,630	$ 27,413
Total Available-for-Sale, at Fair Value	27,732	30,654
One Year or Less, Amortized Cost	14
One Year or Less, Fair Value	14
After One Year through Five Years, Amortized Cost	1,864
After One Year through Five Years, Fair Value	1,943
After Five Years through Ten Years, Amortized Cost	1,017
After Five Years through Ten Years, Fair Value	1,090
After Ten Years, Amortized Cost	21,735
After Ten Years, Fair Value	24,685
Fannie Mae [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total Amortized Cost	5,086	5,330
Total Available-for-Sale, at Fair Value	5,407	5,639
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year through Five Years, Amortized Cost	253
After One Year through Five Years, Fair Value	264
After Five Years through Ten Years, Amortized Cost	208
After Five Years through Ten Years, Fair Value	226
After Ten Years, Amortized Cost	4,625
After Ten Years, Fair Value	4,917
Freddie Mac [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total Amortized Cost	4,842	5,100
Total Available-for-Sale, at Fair Value	5,257	5,528
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year through Five Years, Amortized Cost	260
After One Year through Five Years, Fair Value	274
After Five Years through Ten Years, Amortized Cost	444
After Five Years through Ten Years, Fair Value	485
After Ten Years, Amortized Cost	4,138
After Ten Years, Fair Value	4,498
Ginnie Mae [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total Amortized Cost	398	416
Total Available-for-Sale, at Fair Value	458	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	59
After Five Years through Ten Years, Fair Value	67
After Ten Years, Amortized Cost	338
After Ten Years, Fair Value	390
Alt-A private-label securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total Amortized Cost	3,649	4,638
Total Available-for-Sale, at Fair Value	4,464	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	3,649
After Ten Years, Fair Value	4,464
Subprime private-label securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total Amortized Cost	3,572	4,103
Total Available-for-Sale, at Fair Value	4,608	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	3,572
After Ten Years, Fair Value	4,608
CMBS [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total Amortized Cost	1,318	1,341
Total Available-for-Sale, at Fair Value	1,370	1,397
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year through Five Years, Amortized Cost	1,244
After One Year through Five Years, Fair Value	1,296
After Five Years through Ten Years, Amortized Cost	0
After Five Years through Ten Years, Fair Value	0
After Ten Years, Amortized Cost	74
After Ten Years, Fair Value	74
Mortgage revenue bonds [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total Amortized Cost	3,344	3,859
Total Available-for-Sale, at Fair Value	3,560	4,023
One Year or Less, Amortized Cost	14
One Year or Less, Fair Value	14
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	272
After Five Years through Ten Years, Fair Value	274
After Ten Years, Amortized Cost	2,952
After Ten Years, Fair Value	3,164
Other mortgage-related securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total Amortized Cost	2,421	2,626
Total Available-for-Sale, at Fair Value	2,608	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	34
After Five Years through Ten Years, Fair Value	38
After Ten Years, Amortized Cost	2,387
After Ten Years, Fair Value	$ 2,570