Debt - Summary of Indebtedness (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Short-term debt:
Bank Credit Facility, expires July 30, 2019	$ 0	$ 0
Long-term debt:
Long-term debt	297,536,000	297,469,000
Debt and Capital Lease Obligations	297,536,000	297,469,000
Senior Notes 4.50% [Member]
Long-term debt:
Long-term debt	247,536,000	247,469,000
Unamortized discount	$ 532,000	547,000
Stated rate	4.50%
Face amount	$ 250,000,000
Unamortized debt issuance costs	1,932,000	1,984,000
Federal Home Loan Bank Advances [Member]
Long-term debt:
Long-term debt	$ 50,000,000	$ 50,000,000