Consolidated Income Statements - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues:
Total revenue	$ 1,377,260	$ 1,318,641	$ 1,170,026
Operating expenses:
Selling, general and administrative expenses	417,523	382,389	382,676
Acquisition-related costs	6,014	777	5,093
Depreciation and amortization expenses	74,921	70,501	66,614
Gain on disposition of property, plant and equipment	(1,559)	(1,107)	(2,731)
Foreign exchange loss	1,612	(2,937)	(212)
Total operating expenses	1,114,100	1,095,439	984,837
Operating income	263,160	223,202	185,189
Interest expense	(35,568)	(41,277)	(44,527)
Other income, net	8,296	8,838	11,850
Income before income taxes	235,888	190,763	152,512
Current income tax	56,378	32,797	24,767
Deferred income tax expense	9,152	8,826	6,239
Income tax expense	65,530	41,623	31,006
Net income	170,358	149,140	121,506
Net income attributable to:
Stockholders	170,095	149,039	121,479
Non-controlling interests	263	101	27
Net income	$ 170,358	$ 149,140	$ 121,506
Earnings per share attributable to stockholders:
Basic	$ 1.56	$ 1.37	$ 1.12
Diluted	$ 1.54	$ 1.36	$ 1.11
Weighted average number of shares outstanding:
Basic	109,054,493	108,519,739	108,063,349
Diluted	110,310,984	109,759,123	109,388,236
Service Revenue [Member]
Revenues:
Total revenue	$ 871,596	$ 804,024	$ 749,515
Operating expenses:
Direct expenses	157,296	164,977	159,058
Inventory Sales Revenue [Member]
Revenues:
Total revenue	505,664	514,617	420,511
Operating expenses:
Direct expenses	$ 458,293	$ 480,839	$ 374,339