Debt - Summary of Debt (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Short-term debt
Bank Credit Facility	$ 0	$ 0
Long-term debt
Long-term debt	297,740,000	297,469,000
Total	$ 297,740,000	297,469,000
4.50% Senior Notes
Debt Instrument [Line Items]
Effective Interest Rates	4.50%
Long-term debt
Long-term debt	$ 247,740,000	247,469,000
Principal amount	250,000,000
Unamortized discount	488,000	547,000
Unamortized debt issuance costs	$ 1,772,000	1,984,000
Federal Home Loan Bank borrowing
Debt Instrument [Line Items]
Effective Interest Rates	2.70%
Long-term debt
Long-term debt	$ 50,000,000	$ 50,000,000