Debt - Summary of Debt (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018	Nov. 23, 2015
Short-term debt
Bank Credit Facility	$ 135,000,000	$ 0
Long-term debt
Long-term debt	298,025,000	297,740,000
Total	433,025,000	297,740,000
4.50% Senior Notes
Long-term debt
Long-term debt	$ 248,025,000	247,740,000
Effective Interest Rates	4.50%		4.50%
Principal amount	$ 250,000,000		$ 250,000,000.0
Unamortized discount	426,000	488,000
Unamortized debt issuance costs	1,549,000	1,772,000
Federal Home Loan Bank borrowing
Long-term debt
Long-term debt	$ 50,000,000	$ 50,000,000
Effective Interest Rates	1.99%