Investments in Securities Maturity Information (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	$ 27,413	$ 35,690
Total fair value	30,654	38,171
One Year or Less, Amortized Cost	16
One Year or Less, Fair Value	16
After One Year through Five Years, Amortized Cost	1,965
After One Year through Five Years, Fair Value	2,053
After Five Years through Ten Years, Amortized Cost	1,181
After Five Years through Ten Years, Fair Value	1,258
After Ten Years, Amortized Cost	24,251
After Ten Years, Fair Value	27,327
Weighted average yield (percent)	5.52%
One Year or Less, Weighted Average Yield	6.72%
After One Year through Five Years, Weighted Average Yield	4.61%
After Five Years through Ten Years, Weighted Average Yield	5.97%
After Ten Years, Weighted Average Yield	5.58%
Fannie Mae [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	5,330	6,227
Total fair value	5,639	6,573
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	266
After Five Years through Ten Years, Amortized Cost	255
After Five Years through Ten Years, Fair Value	274
After Ten Years, Amortized Cost	4,822
After Ten Years, Fair Value	5,099
Freddie Mac [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	5,100	6,365
Total fair value	5,528	6,842
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year through Five Years, Amortized Cost	280
After One Year through Five Years, Fair Value	295
After Five Years through Ten Years, Amortized Cost	488
After Five Years through Ten Years, Fair Value	533
After Ten Years, Amortized Cost	4,332
After Ten Years, Fair Value	4,700
Ginnie Mae [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	416	512
Total fair value	476	588
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	62
After Five Years through Ten Years, Fair Value	70
After Ten Years, Amortized Cost	354
After Ten Years, Fair Value	406
Alt-A private-label securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	4,638	6,240
Total fair value	5,678	7,349
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	4,638
After Ten Years, Fair Value	5,678
Subprime private-label securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	4,103	6,232
Total fair value	5,240	7,068
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	4,103
After Ten Years, Fair Value	5,240
CMBS [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	1,341	1,526
Total fair value	1,397	1,606
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year through Five Years, Amortized Cost	1,264
After One Year through Five Years, Fair Value	1,320
After Five Years through Ten Years, Amortized Cost	0
After Five Years through Ten Years, Fair Value	0
After Ten Years, Amortized Cost	77
After Ten Years, Fair Value	77
Mortgage revenue bonds [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	3,859	5,645
Total fair value	4,023	5,256
One Year or Less, Amortized Cost	16
One Year or Less, Fair Value	16
After One Year through Five Years, Amortized Cost	168
After One Year through Five Years, Fair Value	170
After Five Years through Ten Years, Amortized Cost	340
After Five Years through Ten Years, Fair Value	343
After Ten Years, Amortized Cost	3,335
After Ten Years, Fair Value	3,494
Other mortgage-related securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	2,626	2,943
Total fair value	2,673	2,889
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	2
After Five Years through Ten Years, Amortized Cost	36
After Five Years through Ten Years, Fair Value	38
After Ten Years, Amortized Cost	2,590
After Ten Years, Fair Value	$ 2,633