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Debt and Credit Arrangements (Narrative) (Details) - USD ($)		1 Months Ended	12 Months Ended
	Aug. 21, 2025	Jul. 31, 2023	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Debt Instrument [Line Items]
Proceeds from issuance of debt, net			$ 495,000,000	$ 0	$ 0
Repayments Of Debt			61,800,000	0	0
Federal Home Loan Bank Stock			6,100,000	6,100,000
Outstanding borrowings with FHLB			0	0
Line of Credit Facility, Maximum Borrowing Capacity		$ 150,000,000.0
Outstanding borrowings under credit agreement			0	0
Interest Expense Debt			43,200,000	34,100,000	$ 34,100,000
Collateralized Borrowing Program [Member]
Debt Instrument [Line Items]
Securities Held As Collateral At Fair Value			339,200,000	130,900,000
Unsecured Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Unsecured revolving credit facility term		5 years
Senior debentures maturing September 1, 2030 [Member]
Debt Instrument [Line Items]
Aggregate principal amount			$ 300,000,000.0	$ 300,000,000.0
Debt instrument interest rate			2.50%	2.50%
Debt issuance date			Aug. 24, 2020	Aug. 24, 2020
Maturity date			Sep. 01, 2030	Sep. 01, 2030
Senior debentures maturing September 1, 2035 [Member]
Debt Instrument [Line Items]
Aggregate principal amount	$ 500,000,000
Debt instrument interest rate	5.50%
Maturity date	Sep. 01, 2035		Sep. 01, 2035	Sep. 01, 2035
Proceeds from issuance of debt, net	$ 496,700,000
Senior debentures unsecured mature on October 15, 2025 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	7.625%
Maturity date	Oct. 15, 2025		Oct. 15, 2025	Oct. 15, 2025
Repayments Of Debt	$ 61,800,000
Senior debentures unsecured mature on April 15, 2026 [Member]
Debt Instrument [Line Items]
Aggregate principal amount			$ 375,000,000.0	$ 375,000,000.0
Debt instrument interest rate			4.50%	4.50%
Debt issuance date			Apr. 08, 2016	Apr. 08, 2016
Maturity date			Apr. 15, 2026	Apr. 15, 2026
8.207% Subordinated Debentures [Member]
Debt Instrument [Line Items]
Aggregate principal amount			$ 50,100,000	$ 50,100,000
Maturity date			Feb. 03, 2027	Feb. 03, 2027
Interest rate of Series B Subordinated Deferrable Interest Debentures			8.207%	8.207%

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Amount of borrowings as of the balance sheet date from the Federal Home Loan Bank, which are primarily used to cover shortages in the required reserve balance and liquidity shortages.

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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 230
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 15
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-15

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+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-SubTopic 10
-Section 50
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Stated interest rate of the subordinated debt.

+ References

No definition available.

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