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Debt, Financing Arrangements, and Leases Debt Disclosure (Details) - USD ($)
Dec. 31, 2020
Dec. 31, 2019
Debt Instrument [Line Items]    
Long-term Debt $ 578,018,000 $ 577,045,000
Debt issuance costs (3,581,000) (4,083,000)
Unamortized debt discount (3,401,000) (3,872,000)
Property pledged as collateral 0  
Long-term Debt, Fiscal Year Maturity [Abstract]    
2021 0  
2022 0  
2023 0  
2024 0  
2025 85,000,000  
7.125 percent unsecured debentures due 2025 [Member]    
Debt Instrument [Line Items]    
Long-term Debt 85,000,000 85,000,000
4.0% unsecured debentures due 2027 [Member]    
Debt Instrument [Line Items]    
Long-term Debt $ 500,000,000 $ 500,000,000