Debt and Credit Arrangements (Narrative) (Details) - USD ($)	1 Months Ended	12 Months Ended
	Jul. 31, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Debt Instrument [Line Items]
Federal Home Loan Bank Stock		$ 6,100,000	$ 6,000,000
Outstanding borrowings with FHLB		0	0
Line of Credit Facility, Maximum Borrowing Capacity	$ 150,000,000.0
Termination of credit agreement	$ 200,000,000
Outstanding borrowings under credit agreement		0
Interest Expense Debt		34,100,000	34,100,000	$ 34,000,000.0
Collateralized Borrowing Program [Member]
Debt Instrument [Line Items]
Securities Held As Collateral At Fair Value		153,000,000.0	$ 132,500,000
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
Debt instrument interest rate		2.50%
Debt issuance date		Aug. 24, 2020	Aug. 24, 2020
Maturity date		Sep. 01, 2030	Sep. 01, 2030
Long-term Debt, Gross		$ 300,000,000.0	$ 300,000,000.0
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
Long-term Debt, Gross		$ 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
Maturity date		Apr. 15, 2026	Apr. 15, 2026
Long-term Debt, Gross		$ 375,000,000.0	$ 375,000,000.0
8.207% Subordinated Debentures [Member]
Debt Instrument [Line Items]
Aggregate principal amount		$ 50,100,000	$ 50,100,000
Maturity date		Feb. 03, 2027
Interest rate of Series B Subordinated Deferrable Interest Debentures		8.207%	8.207%