Supplemental Cash Flow Information (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Cash Paid During the Period For:
Income taxes	$ 4,343	$ 7,542
Interest, net	2,199	1,624
Non-Cash Financing and Investing Transactions:
Employee stock purchase plan	261	281
Equipment acquired under capital leases	424	465
Shares tendered in payment of exercise price of stock options	46	$ 0
Proceeds from income tax refunds	$ 6,800