Investments in Securities Maturity Information (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	$ 3,508	$ 4,058
Total fair value	3,888	4,843
One Year or Less, Amortized Cost	10
One Year or Less, Fair Value	10
After One Year through Five Years, Amortized Cost	58
After One Year through Five Years, Fair Value	58
After Five Years through Ten Years, Amortized Cost	205
After Five Years through Ten Years, Fair Value	216
After Ten Years, Amortized Cost	3,235
After Ten Years, Fair Value	3,604
Fannie Mae [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	1,940	2,044
Total fair value	1,986	2,119
One Year or Less, Amortized Cost	1
One Year or Less, Fair Value	1
After One Year through Five Years, Amortized Cost	11
After One Year through Five Years, Fair Value	11
After Five Years through Ten Years, Amortized Cost	82
After Five Years through Ten Years, Fair Value	87
After Ten Years, Amortized Cost	1,846
After Ten Years, Fair Value	1,887
Other agency [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	305	332
Total fair value	327	357
One Year or Less, Amortized Cost	1
One Year or Less, Fair Value	1
After One Year through Five Years, Amortized Cost	11
After One Year through Five Years, Fair Value	11
After Five Years through Ten Years, Amortized Cost	53
After Five Years through Ten Years, Fair Value	57
After Ten Years, Amortized Cost	240
After Ten Years, Fair Value	258
Alt-A and subprime private-label securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	376	662
Total fair value	671	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	0
After Five Years through Ten Years, Fair Value	0
After Ten Years, Amortized Cost	376
After Ten Years, Fair Value	671
CMBS [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	5	15
Total fair value	5	15
One Year or Less, Amortized Cost	5
One Year or Less, Fair Value	5
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	536	655
Total fair value	539	671
One Year or Less, Amortized Cost	3
One Year or Less, Fair Value	3
After One Year through Five Years, Amortized Cost	36
After One Year through Five Years, Fair Value	36
After Five Years through Ten Years, Amortized Cost	65
After Five Years through Ten Years, Fair Value	67
After Ten Years, Amortized Cost	432
After Ten Years, Fair Value	433
Other mortgage-related securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	346	350
Total fair value	360	$ 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	5
After Five Years through Ten Years, Fair Value	5
After Ten Years, Amortized Cost	341
After Ten Years, Fair Value	$ 355