Debt - Summary of Debt (Details) - USD ($)	Dec. 31, 2020	Dec. 31, 2019	Nov. 23, 2015
Short-term debt
Bank Credit Facility	$ 135,000,000	$ 135,000,000
Long-term debt
Long-term debt	302,323,000	298,025,000
Total	437,323,000	433,025,000
4.50% Senior Notes
Long-term debt
Long-term debt	$ 248,323,000	$ 248,025,000
Effective Interest Rates	4.50%	4.50%	4.50%
Principal amount	$ 250,000,000	$ 250,000,000	$ 250,000,000.0
Unamortized discount	426,000	362,000
Unamortized debt issuance costs	1,549,000	1,315,000
Federal Home Loan Bank borrowing
Long-term debt
Long-term debt	$ 54,000,000	$ 50,000,000