Investments in Securities Maturity Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	$ 3,079	$ 4,058
Total fair value	3,429	4,843
One Year or Less, Amortized Cost	2
One Year or Less, Fair Value	2
After One Year through Five Years, Amortized Cost	67
After One Year through Five Years, Fair Value	69
After Five Years through Ten Years, Amortized Cost	159
After Five Years through Ten Years, Fair Value	166
After Ten Years, Amortized Cost	2,851
After Ten Years, Fair Value	$ 3,192
Weighted average yield	6.99%
One Year or Less, weighted average yield	4.37%
After One Year through Five Years, weighted average yield	5.68%
After Five Years through Ten Years, weighted average yield	6.24%
After Ten Years, weighted average yield	7.07%
Fannie Mae [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	$ 1,754	2,044
Total fair value	1,797	2,119
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year through Five Years, Amortized Cost	13
After One Year through Five Years, Fair Value	14
After Five Years through Ten Years, Amortized Cost	72
After Five Years through Ten Years, Fair Value	76
After Ten Years, Amortized Cost	1,669
After Ten Years, Fair Value	1,707
Other agency [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	239	332
Total fair value	256	357
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year through Five Years, Amortized Cost	24
After One Year through Five Years, Fair Value	25
After Five Years through Ten Years, Amortized Cost	25
After Five Years through Ten Years, Fair Value	27
After Ten Years, Amortized Cost	190
After Ten Years, Fair Value	204
Alt-A and subprime private-label securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	325	662
Total fair value	592	1,314
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	1
After Five Years through Ten Years, Fair Value	1
After Ten Years, Amortized Cost	324
After Ten Years, Fair Value	591
CMBS [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost		15
Total fair value		15
Mortgage revenue bonds [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	425	655
Total fair value	434	671
One Year or Less, Amortized Cost	2
One Year or Less, Fair Value	2
After One Year through Five Years, Amortized Cost	30
After One Year through Five Years, Fair Value	30
After Five Years through Ten Years, Amortized Cost	55
After Five Years through Ten Years, Fair Value	56
After Ten Years, Amortized Cost	338
After Ten Years, Fair Value	346
Other mortgage-related securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	336	350
Total fair value	350	$ 367
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	6
After Five Years through Ten Years, Fair Value	6
After Ten Years, Amortized Cost	330
After Ten Years, Fair Value	$ 344