Investments in Securities - Available-for-sale Securities 1 (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 802.0	$ 827.0
Allowance for credit losses	0.0	0.0
Gross unrealized gains	18.0	24.0
Gross unrealized losses	(14.0)	(14.0)
Available-for-sale, at fair value	806.0	837.0
Accrued interest	2.0	2.0
Maximum
Debt Securities, Available-for-sale [Line Items]
Allowance for credit losses	(0.5)	(0.5)
Fannie Mae
Debt Securities, Available-for-sale [Line Items]
Amortized cost	478.0	492.0
Allowance for credit losses	0.0	0.0
Gross unrealized gains	10.0	14.0
Gross unrealized losses	(12.0)	(11.0)
Available-for-sale, at fair value	476.0	495.0
Other agency
Debt Securities, Available-for-sale [Line Items]
Amortized cost	11.0	12.0
Allowance for credit losses	0.0	0.0
Gross unrealized gains	1.0	0.0
Gross unrealized losses	0.0	0.0
Available-for-sale, at fair value	12.0	12.0
Alt-A and subprime private-label securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	3.0	3.0
Allowance for credit losses	0.0	0.0
Gross unrealized gains	2.0	2.0
Gross unrealized losses	0.0	0.0
Available-for-sale, at fair value	5.0	5.0
Mortgage revenue bonds
Debt Securities, Available-for-sale [Line Items]
Amortized cost	139.0	142.0
Allowance for credit losses	0.0	0.0
Gross unrealized gains	4.0	5.0
Gross unrealized losses	(2.0)	(3.0)
Available-for-sale, at fair value	141.0	144.0
Other mortgage-related securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	171.0	178.0
Allowance for credit losses	0.0	0.0
Gross unrealized gains	1.0	3.0
Gross unrealized losses	0.0	0.0
Available-for-sale, at fair value	$ 172.0	$ 181.0

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, after allowance for credit loss, of accrued interest on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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