Condensed Consolidated Income Statements - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Revenue:
Total revenue	$ 396,361,000	$ 389,050,000	$ 727,916,000	$ 662,305,000
Operating expenses:
Selling, general and administrative expenses	111,819,000	100,632,000	227,897,000	199,017,000
Acquisition-related costs	3,049,000		5,971,000
Depreciation and amortization expenses	21,935,000	17,857,000	43,005,000	37,150,000
Gain on disposition of property, plant and equipment	(175,000)	(1,213,000)	(243,000)	(1,260,000)
Foreign exchange loss	151,000	392,000	428,000	994,000
Total operating expenses	306,844,000	300,250,000	593,897,000	539,423,000
Operating income	89,517,000	88,800,000	134,019,000	122,882,000
Interest expense	(8,867,000)	(8,882,000)	(17,813,000)	(18,064,000)
Other income, net	1,196,000	857,000	2,198,000	4,434,000
Income before income taxes	81,846,000	80,775,000	118,404,000	109,252,000
Income tax expense	21,065,000	27,656,000	29,484,000	33,304,000
Net income	60,781,000	53,119,000	88,920,000	75,948,000
Net income attributable to:
Stockholders	60,749,000	53,043,000	88,937,000	75,851,000
Non-controlling interests	32,000	76,000	(17,000)	97,000
Net income	$ 60,781,000	$ 53,119,000	$ 88,920,000	$ 75,948,000
Earnings per share attributable to stockholders:
Basic	$ 0.55	$ 0.49	$ 0.81	$ 0.70
Diluted	$ 0.55	$ 0.49	$ 0.80	$ 0.69
Weighted average number of shares outstanding:
Basic	110,311,615	108,387,490	110,144,229	108,818,903
Diluted	111,334,184	109,323,343	111,302,711	109,903,808
Service Revenue [Member]
Revenue:
Total revenue	$ 252,748,000	$ 234,139,000	$ 458,778,000	$ 417,262,000
Operating expenses:
Direct expenses	39,042,000	39,448,000	75,069,000	78,803,000
Inventory Sales Revenue [Member]
Revenue:
Total revenue	143,613,000	154,911,000	269,138,000	245,043,000
Operating expenses:
Direct expenses	$ 131,023,000	$ 143,134,000	$ 241,770,000	$ 224,719,000