Investments in Securities Maturity Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	$ 18,141	$ 27,413
Total fair value	20,230	30,654
One Year or Less, Amortized Cost	290
One Year or Less, Fair Value	292
After One Year through Five Years, Amortized Cost	1,357
After One Year through Five Years, Fair Value	1,387
After Five Years through Ten Years, Amortized Cost	716
After Five Years through Ten Years, Fair Value	763
After Ten Years, Amortized Cost	15,778
After Ten Years, Fair Value	$ 17,788
Weighted average yield (percent)	4.94%
One Year or Less, Weighted Average Yield	4.36%
After One Year through Five Years, Weighted Average Yield	4.51%
After Five Years through Ten Years, Weighted Average Yield	6.28%
After Ten Years, Weighted Average Yield	4.93%
Fannie Mae [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	$ 4,008	5,330
Total fair value	4,221	5,639
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year through Five Years, Amortized Cost	160
After One Year through Five Years, Fair Value	164
After Five Years through Ten Years, Amortized Cost	114
After Five Years through Ten Years, Fair Value	123
After Ten Years, Amortized Cost	3,734
After Ten Years, Fair Value	3,934
Freddie Mac [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	4,000	5,100
Total fair value	4,299	5,528
One Year or Less, Amortized Cost	1
One Year or Less, Fair Value	1
After One Year through Five Years, Amortized Cost	185
After One Year through Five Years, Fair Value	192
After Five Years through Ten Years, Amortized Cost	310
After Five Years through Ten Years, Fair Value	337
After Ten Years, Amortized Cost	3,504
After Ten Years, Fair Value	3,769
Ginnie Mae [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	343	416
Total fair value	391	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	61
After Five Years through Ten Years, Fair Value	68
After Ten Years, Amortized Cost	281
After Ten Years, Fair Value	322
Alt-A private-label securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	2,029	4,638
Total fair value	2,678	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	2,029
After Ten Years, Fair Value	2,678
Subprime private-label securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	2,526	4,103
Total fair value	3,281	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	2,526
After Ten Years, Fair Value	3,281
CMBS [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	1,235	1,341
Total fair value	1,255	1,397
One Year or Less, Amortized Cost	278
One Year or Less, Fair Value	280
After One Year through Five Years, Amortized Cost	899
After One Year through Five Years, Fair Value	917
After Five Years through Ten Years, Amortized Cost	0
After Five Years through Ten Years, Fair Value	0
After Ten Years, Amortized Cost	58
After Ten Years, Fair Value	58
Mortgage revenue bonds [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	2,639	3,859
Total fair value	2,701	4,023
One Year or Less, Amortized Cost	11
One Year or Less, Fair Value	11
After One Year through Five Years, Amortized Cost	112
After One Year through Five Years, Fair Value	113
After Five Years through Ten Years, Amortized Cost	202
After Five Years through Ten Years, Fair Value	205
After Ten Years, Amortized Cost	2,314
After Ten Years, Fair Value	2,372
Other mortgage-related securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	1,361	2,626
Total fair value	1,404	$ 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	29
After Five Years through Ten Years, Fair Value	30
After Ten Years, Amortized Cost	1,332
After Ten Years, Fair Value	$ 1,374