Investments in Securities Maturity Information (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	$ 3,338	$ 4,058
Total fair value	3,723	4,843
One Year or Less, Amortized Cost	6
One Year or Less, Fair Value	6
After One Year through Five Years, Amortized Cost	56
After One Year through Five Years, Fair Value	56
After Five Years through Ten Years, Amortized Cost	191
After Five Years through Ten Years, Fair Value	199
After Ten Years, Amortized Cost	3,085
After Ten Years, Fair Value	3,462
Fannie Mae [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	1,866	2,044
Total fair value	1,904	2,119
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
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	80
After Five Years through Ten Years, Fair Value	85
After Ten Years, Amortized Cost	1,774
After Ten Years, Fair Value	1,807
Other agency [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	279	332
Total fair value	298	357
One Year or Less, Amortized Cost	2
One Year or Less, Fair Value	2
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	44
After Five Years through Ten Years, Fair Value	47
After Ten Years, Amortized Cost	222
After Ten Years, Fair Value	238
Alt-A and subprime private-label securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	357	662
Total fair value	649	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	357
After Ten Years, Fair Value	649
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	494	655
Total fair value	506	671
One Year or Less, Amortized Cost	4
One Year or Less, Fair Value	4
After One Year through Five Years, Amortized Cost	33
After One Year through Five Years, Fair Value	33
After Five Years through Ten Years, Amortized Cost	61
After Five Years through Ten Years, Fair Value	61
After Ten Years, Amortized Cost	396
After Ten Years, Fair Value	408
Other mortgage-related securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	342	350
Total fair value	366	$ 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	336
After Ten Years, Fair Value	$ 360