Investments in Securities - Available-for-sale Securities 1 (Details) - USD ($) $ in Millions	Jun. 30, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 764.0	$ 827.0
Allowance for credit losses	0.0	0.0
Gross unrealized gains	17.0	24.0
Gross unrealized losses	(16.0)	(14.0)
Available-for-sale, at fair value	765.0	837.0
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	461.0	492.0
Allowance for credit losses	0.0	0.0
Gross unrealized gains	11.0	14.0
Gross unrealized losses	(14.0)	(11.0)
Available-for-sale, at fair value	458.0	495.0
Other agency
Debt Securities, Available-for-sale [Line Items]
Amortized cost	10.0	12.0
Allowance for credit losses	0.0	0.0
Gross unrealized gains	0.0	0.0
Gross unrealized losses	0.0	0.0
Available-for-sale, at fair value	10.0	12.0
Other mortgage-related securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	293.0	323.0
Allowance for credit losses	0.0	0.0
Gross unrealized gains	6.0	10.0
Gross unrealized losses	(2.0)	(3.0)
Available-for-sale, at fair value	$ 297.0	$ 330.0

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).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27161-111563

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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).

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
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-URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27161-111563

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=124258926&loc=SL82898722-210454

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Amortized cost excluding accrued interest, before allowance for credit loss, of 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 excluding accrued interest, of 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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