Investments in Securities Maturity Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	$ 4,058	$ 7,261
Total fair value	4,843	8,363
One Year or Less, Amortized Cost	34
One Year or Less, Fair Value	34
After One Year through Five Years, Amortized Cost	102
After One Year through Five Years, Fair Value	102
After Five Years through Ten Years, Amortized Cost	204
After Five Years through Ten Years, Fair Value	214
After Ten Years, Amortized Cost	3,718
After Ten Years, Fair Value	$ 4,493
Weighted average yield	6.84%
One Year or Less, weighted average yield	4.18%
After One Year through Five Years, weighted average yield	6.60%
After Five Years through Ten Years, weighted average yield	6.59%
After Ten Years, weighted average yield	6.88%
Fannie Mae [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	$ 2,044	2,445
Total fair value	2,119	2,554
One Year or Less, Amortized Cost	2
One Year or Less, Fair Value	2
After One Year through Five Years, Amortized Cost	12
After One Year through Five Years, Fair Value	12
After Five Years through Ten Years, Amortized Cost	59
After Five Years through Ten Years, Fair Value	63
After Ten Years, Amortized Cost	1,971
After Ten Years, Fair Value	2,042
Other agency [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	332	508
Total fair value	357	547
One Year or Less, Amortized Cost	2
One Year or Less, Fair Value	2
After One Year through Five Years, Amortized Cost	15
After One Year through Five Years, Fair Value	15
After Five Years through Ten Years, Amortized Cost	57
After Five Years through Ten Years, Fair Value	61
After Ten Years, Amortized Cost	258
After Ten Years, Fair Value	279
Alt-A and subprime private-label securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	662	1,817
Total fair value	1,314	2,709
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	662
After Ten Years, Fair Value	1,314
CMBS [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	15	815
Total fair value	15	819
One Year or Less, Amortized Cost	15
One Year or Less, Fair Value	15
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	0
After Ten Years, Fair Value	0
Mortgage revenue bonds [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	655	1,245
Total fair value	671	1,272
One Year or Less, Amortized Cost	15
One Year or Less, Fair Value	15
After One Year through Five Years, Amortized Cost	75
After One Year through Five Years, Fair Value	75
After Five Years through Ten Years, Amortized Cost	83
After Five Years through Ten Years, Fair Value	85
After Ten Years, Amortized Cost	482
After Ten Years, Fair Value	496
Other mortgage-related securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	350	431
Total fair value	367	$ 462
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	5
After Five Years through Ten Years, Fair Value	5
After Ten Years, Amortized Cost	345
After Ten Years, Fair Value	$ 362