Supplemental Cash Flow Information - Details of Supplemental Cash Flow Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash paid during the year for:
Income taxes, net	$ 24,330	$ 21,324	$ 25,395
Interest, net	3,518	2,101	1,609
Non-Cash Financing and Investing Transactions:
Receivable for sale of Global's assets	1,979	0	0
Equipment acquired under capital leases	937	1,153	1,470
Unsettled repurchases of common stock	898	935	1,012
Employee stock purchase plan	522	669	578
Shares tendered in payment of exercise price of stock options	$ 0	$ 63	$ 0