Debt (Details)	1 Months Ended	3 Months Ended
	May 31, 2013 USD ($)	Mar. 31, 2018 USD ($) installment	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Principal Payments
Total debt		$ 270,300,000		$ 255,500,000
Deferred financing costs		(2,500,000)		(2,800,000)
Less: Debt payable within one year		1,400,000		1,400,000
Long-term debt		268,900,000		$ 254,100,000
Scheduled debt repayment		300,000	$ 300,000
Long-term debt borrowings		14,000,000
Repayments of long term debt			$ 5,200,000
Global Revolving Credit Facilities (variable rates) due December 2019
Principal Payments
Letters of credit outstanding		900,000
Available credit		$ 79,200,000
Weighted-average interest rate (as a percent)		2.60%		2.70%
Senior notes | 2021 Senior Notes (5.25% fixed rate) due May 2021
Principal Payments
Total debt		$ 175,000,000		$ 175,000,000
Debt instrument, interest rate, stated percentage		5.25%
Total term of notes	8 years
Face amount	$ 175,000,000
Line of credit | Global Revolving Credit Facilities (variable rates) due December 2019
Principal Payments
Total debt		$ 91,500,000		76,900,000
Secured debt | German loan agreement (2.45% fixed rate) due in 32 equal quarterly installments ending September 2022
Principal Payments
Total debt		$ 6,300,000		$ 6,400,000
Debt instrument, interest rate, stated percentage		2.45%
Debt instrument, due number of equal quarterly installments | installment		32