Investments in Securities Maturity Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	$ 1,606	$ 2,281
Total fair value	1,697	2,404
One Year or Less, Amortized Cost	1
One Year or Less, Fair Value	1
After One Year through Five Years, Amortized Cost	32
After One Year through Five Years, Fair Value	34
After Five Years through Ten Years, Amortized Cost	146
After Five Years through Ten Years, Fair Value	162
After Ten Years, Amortized Cost	1,424
After Ten Years, Fair Value	$ 1,500
Weighted average yield	5.27%
One Year or Less, weighted average yield	5.90%
After One Year through Five Years, weighted average yield	6.54%
After Five Years through Ten Years, weighted average yield	6.11%
After Ten Years, weighted average yield	5.16%
Carrying Value [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	$ 1,603
Fannie Mae [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	1,094	1,445
Total fair value	1,168	1,520
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year through Five Years, Amortized Cost	5
After One Year through Five Years, Fair Value	6
After Five Years through Ten Years, Amortized Cost	96
After Five Years through Ten Years, Fair Value	108
After Ten Years, Amortized Cost	993
After Ten Years, Fair Value	1,054
Other agency [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	59	183
Total fair value	65	198
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	9
After Five Years through Ten Years, Fair Value	10
After Ten Years, Amortized Cost	50
After Ten Years, Fair Value	55
Alt-A and subprime private-label securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	4	34
Total fair value	6	57
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	3
After Five Years through Ten Years, Fair Value	3
After Ten Years, Amortized Cost	1
After Ten Years, Fair Value	3
Mortgage revenue bonds [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	211	309
Total fair value	216	315
One Year or Less, Amortized Cost	1
One Year or Less, Fair Value	1
After One Year through Five Years, Amortized Cost	27
After One Year through Five Years, Fair Value	28
After Five Years through Ten Years, Amortized Cost	19
After Five Years through Ten Years, Fair Value	20
After Ten Years, Amortized Cost	161
After Ten Years, Fair Value	167
Mortgage revenue bonds [Member] | Carrying Value [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	208
Other mortgage-related securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	238	310
Total fair value	242	$ 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	19
After Five Years through Ten Years, Fair Value	21
After Ten Years, Amortized Cost	219
After Ten Years, Fair Value	$ 221