Investments in Securities - Schedule of Available-for-sale Securities Reconciliation (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 827.0	$ 1,606.0
Allowance for credit losses	0.0	(3.0)
Gross unrealized gains	24.0	106.0
Gross unrealized losses	(14.0)	(12.0)
Available-for-sale	837.0	1,697.0
Accrued interest	2.0	6.0
Maximum
Debt Securities, Available-for-sale [Line Items]
Allowance for credit losses	(0.5)
Fannie Mae
Debt Securities, Available-for-sale [Line Items]
Amortized cost	492.0	1,094.0
Allowance for credit losses	0.0	0.0
Gross unrealized gains	14.0	86.0
Gross unrealized losses	(11.0)	(12.0)
Available-for-sale	495.0	1,168.0
Other agency
Debt Securities, Available-for-sale [Line Items]
Amortized cost	12.0	59.0
Allowance for credit losses	0.0	0.0
Gross unrealized gains	0.0	6.0
Gross unrealized losses	0.0	0.0
Available-for-sale	12.0	65.0
Private-label and other mortgage securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	3.0	4.0
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	5.0	6.0
Mortgage revenue bonds
Debt Securities, Available-for-sale [Line Items]
Amortized cost	142.0	211.0
Allowance for credit losses	0.0	(3.0)
Gross unrealized gains	5.0	8.0
Gross unrealized losses	(3.0)	0.0
Available-for-sale	144.0	216.0
Other mortgage-related securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	178.0	238.0
Allowance for credit losses	0.0	0.0
Gross unrealized gains	3.0	4.0
Gross unrealized losses	0.0	0.0
Available-for-sale	$ 181.0	$ 242.0

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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