Debt and Credit Arrangements (Narrative) (Details) - USD ($)	12 Months Ended
Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Federal Home Loan Bank Stock	$ 6,000,000.0	$ 2,800,000
Outstanding borrowings with FHLB	0	0
Line of Credit Facility, Maximum Borrowing Capacity	$ 200,000,000.0
Line Of Credit Facility Maturity Month And Year	2024-04
Outstanding borrowings under credit agreement	$ 0
Interest Expense Debt	34,100,000	34,000,000.0	$ 37,100,000
Collateralized Borrowing Program [Member]
Debt Instrument [Line Items]
Securities Held As Collateral At Fair Value	132,500,000	$ 106,600,000
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%

Line Of Credit Facility Maturity Month And Year

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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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Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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Face (par) amount of debt instrument at time of issuance.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Date the debt instrument was issued, in YYYY-MM-DD format.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Date when the debt instrument is scheduled to be fully repaid, in YYYY-MM-DD format.

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Federal Home Loan Bank (FHLB) stock represents an equity interest in a FHLB. It does not have a readily determinable fair value because its ownership is restricted and it lacks a market (liquidity).

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Amount of the cost of borrowed funds accounted for as interest expense for debt.

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The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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The carrying amount (fair value) as of the balance sheet date of securities held that had been pledged by counterparties under financing and lending arrangements, pursuant to which the entity has the right by agreement or custom to sell or re-pledge such securities.

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Stated interest rate of the subordinated debt.

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