Debt - Summary of Debt (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Short-term debt
Bank Credit Facility	$ 0	$ 0
Long-term debt
Long-term debt	297,469,000	247,209,000
Total	$ 297,469,000	247,209,000
4.50% Senior Notes
Debt Instrument [Line Items]
Effective Interest Rates	4.50%
Long-term debt
Long-term debt	$ 247,469,000	247,209,000
Principal amount	250,000,000
Unamortized discount	547,000	603,000
Unamortized debt issuance costs	$ 1,984,000	2,188,000
Federal Home Loan Bank borrowing
Debt Instrument [Line Items]
Effective Interest Rates	1.57%
Long-term debt
Long-term debt	$ 50,000,000	$ 0