Investments in Securities Maturity Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	$ 2,281	$ 3,079
Total fair value	2,404	3,429
One Year or Less, Amortized Cost	2
One Year or Less, Fair Value	2
After One Year through Five Years, Amortized Cost	64
After One Year through Five Years, Fair Value	66
After Five Years through Ten Years, Amortized Cost	175
After Five Years through Ten Years, Fair Value	190
After Ten Years, Amortized Cost	2,040
After Ten Years, Fair Value	$ 2,146
Weighted average yield	6.48%
One Year or Less, weighted average yield	5.51%
After One Year through Five Years, weighted average yield	6.30%
After Five Years through Ten Years, weighted average yield	6.19%
After Ten Years, weighted average yield	6.51%
Fannie Mae [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	$ 1,445	1,754
Total fair value	1,520	1,797
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year through Five Years, Amortized Cost	15
After One Year through Five Years, Fair Value	16
After Five Years through Ten Years, Amortized Cost	95
After Five Years through Ten Years, Fair Value	104
After Ten Years, Amortized Cost	1,335
After Ten Years, Fair Value	1,400
Other agency [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	183	239
Total fair value	198	256
One Year or Less, Amortized Cost	0
One Year or Less, Fair Value	0
After One Year through Five Years, Amortized Cost	18
After One Year through Five Years, Fair Value	18
After Five Years through Ten Years, Amortized Cost	24
After Five Years through Ten Years, Fair Value	27
After Ten Years, Amortized Cost	141
After Ten Years, Fair Value	153
Alt-A and subprime private-label securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	34	325
Total fair value	57	592
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	31
After Ten Years, Fair Value	54
Mortgage revenue bonds [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	309	425
Total fair value	315	434
One Year or Less, Amortized Cost	2
One Year or Less, Fair Value	2
After One Year through Five Years, Amortized Cost	31
After One Year through Five Years, Fair Value	32
After Five Years through Ten Years, Amortized Cost	29
After Five Years through Ten Years, Fair Value	30
After Ten Years, Amortized Cost	247
After Ten Years, Fair Value	251
Other mortgage-related securities [Member]
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity
Total amortized cost	310	336
Total fair value	314	$ 350
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	24
After Five Years through Ten Years, Fair Value	26
After Ten Years, Amortized Cost	286
After Ten Years, Fair Value	$ 288