Investments in Securities Maturity Information (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	$ 3,194	$ 4,058
Total fair value	3,537	4,843
One Year or Less, Amortized Cost	5
One Year or Less, Fair Value	5
After One Year through Five Years, Amortized Cost	55
After One Year through Five Years, Fair Value	56
After Five Years through Ten Years, Amortized Cost	187
After Five Years through Ten Years, Fair Value	194
After Ten Years, Amortized Cost	2,947
After Ten Years, Fair Value	3,282
Fannie Mae [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	1,808	2,044
Total fair value	1,828	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	13
After Five Years through Ten Years, Amortized Cost	77
After Five Years through Ten Years, Fair Value	81
After Ten Years, Amortized Cost	1,718
After Ten Years, Fair Value	1,734
Other agency [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	256	332
Total fair value	272	357
One Year or Less, Amortized Cost	2
One Year or Less, Fair Value	1
After One Year through Five Years, Amortized Cost	9
After One Year through Five Years, Fair Value	10
After Five Years through Ten Years, Amortized Cost	42
After Five Years through Ten Years, Fair Value	45
After Ten Years, Amortized Cost	203
After Ten Years, Fair Value	216
Alt-A and subprime private-label securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	337	662
Total fair value	617	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	337
After Ten Years, Fair Value	617
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	454	655
Total fair value	464	671
One Year or Less, Amortized Cost	3
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	62
After Ten Years, Amortized Cost	357
After Ten Years, Fair Value	365
Other mortgage-related securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	339	350
Total fair value	356	$ 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	7
After Five Years through Ten Years, Fair Value	6
After Ten Years, Amortized Cost	332
After Ten Years, Fair Value	$ 350