Debt - Summary of Indebtedness (Details) - USD ($)	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Short-term debt:
Bank Credit Facility, expires July 30, 2019	$ 0	$ 0
Long-term debt:
Long-term debt	$ 247,403,000	247,209,000
Expiration date	Jul. 30, 2019
4.50% Senior Notes
Long-term debt:
Long-term debt	$ 247,403,000	247,209,000
Senior Notes 2025 | 4.50% Senior Notes
Long-term debt:
Unamortized discount	$ 561,000	603,000
Stated rate	4.50%
Face amount	$ 250,000,000
Maturity date	Dec. 01, 2025
Unamortized debt issuance costs	$ 2,036,000	$ 2,188,000