Investments in Securities - Maturity Information (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Total Carrying Amount
Total Carrying Amount, Net	$ 873
One Year or Less	1
After One Year Through Five Years	31
After Five Years Through Ten Years	28
After Ten Years	813
Total Fair Value
Total Fair Value	886	$ 1,697
One Year or Less	1
After One Year Through Five Years	32
After Five Years Through Ten Years	29
After Ten Years	824
Fannie Mae
Total Carrying Amount
Total Carrying Amount, Net	510
One Year or Less	0
After One Year Through Five Years	2
After Five Years Through Ten Years	12
After Ten Years	496
Total Fair Value
Total Fair Value	518	1,168
One Year or Less	0
After One Year Through Five Years	2
After Five Years Through Ten Years	13
After Ten Years	503
Other agency
Total Carrying Amount
Total Carrying Amount, Net	14
One Year or Less	0
After One Year Through Five Years	0
After Five Years Through Ten Years	1
After Ten Years	13
Total Fair Value
Total Fair Value	15	65
One Year or Less	0
After One Year Through Five Years	0
After Five Years Through Ten Years	1
After Ten Years	14
Alt-A and subprime private-label securities
Total Carrying Amount
Total Carrying Amount, Net	3
One Year or Less	0
After One Year Through Five Years	0
After Five Years Through Ten Years	2
After Ten Years	1
Total Fair Value
Total Fair Value	3	6
One Year or Less	0
After One Year Through Five Years	0
After Five Years Through Ten Years	2
After Ten Years	1
Mortgage revenue bonds
Total Carrying Amount
Total Carrying Amount, Net	161
One Year or Less	1
After One Year Through Five Years	29
After Five Years Through Ten Years	10
After Ten Years	121
Total Fair Value
Total Fair Value	164	216
One Year or Less	1
After One Year Through Five Years	30
After Five Years Through Ten Years	9
After Ten Years	124
Other mortgage-related securities
Total Carrying Amount
Total Carrying Amount, Net	185
One Year or Less	0
After One Year Through Five Years	0
After Five Years Through Ten Years	3
After Ten Years	182
Total Fair Value
Total Fair Value	186	$ 242
One Year or Less	0
After One Year Through Five Years	0
After Five Years Through Ten Years	4
After Ten Years	$ 182