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Debt and Credit Arrangements (Narrative) (Details) - USD ($)
12 Months Ended
Sep. 19, 2020
Jan. 02, 2019
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Debt Instrument [Line Items]          
Proceeds from debt borrowings, net     $ 296,400,000    
Unamortized debt issuance costs     6,100,000 $ 9,300,000  
Repayment of FHLB advance   $ 125,000,000.0      
FHLB due date   2029      
FHLB Interest rate   5.50%      
Prepayment fees   $ 26,000,000      
Federal Home Loan Bank Stock     2,600,000 2,100,000  
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     37,100,000 37,500,000 $ 45,100,000
Collateralized Borrowing Program [Member]          
Debt Instrument [Line Items]          
Securities Held As Collateral At Fair Value     103,900,000 $ 94,000,000.0  
Senior debentures maturing September 1, 2030 [Member]          
Debt Instrument [Line Items]          
Aggregate principal amount     $ 300,000,000.0    
Debt issuance date     Aug. 24, 2020    
Debt instrument interest rate     2.50%    
Maturity date     Sep. 01, 2030 Sep. 01, 2030  
Proceeds from debt borrowings, net     $ 296,400,000    
Long-term Debt, Gross     $ 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 $ 62,600,000  
Subordinated debentures maturing March 30, 2053 [Member]          
Debt Instrument [Line Items]          
Aggregate principal amount $ 175,000,000.0        
Debt instrument interest rate 6.35%        
Maturity date Mar. 30, 2053   Mar. 30, 2053 Mar. 30, 2053  
Unamortized debt issuance costs $ 6,100,000        
Long-term Debt, Gross       $ 175,000,000.0  
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 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%