Condensed Consolidated Income Statements - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Revenues:
Total revenue	$ 331,542,000	$ 289,796,000	$ 993,847,000	$ 986,447,000
Operating expenses:
Selling, general and administrative expenses	110,186,000	93,691,000	309,203,000	286,589,000
Acquisition-related costs		45,000		752,000
Depreciation and amortization expenses	18,436,000	17,692,000	55,586,000	51,919,000
Gain on disposition of property, plant and equipment	(276,000)	(821,000)	(1,536,000)	(1,071,000)
Foreign exchange loss	336,000	237,000	1,330,000	1,118,000
Total operating expenses	264,158,000	249,636,000	803,581,000	834,729,000
Operating income	67,384,000	40,160,000	190,266,000	151,718,000
Interest expense	(8,737,000)	(10,090,000)	(26,801,000)	(31,023,000)
Other income, net	2,280,000	1,962,000	6,714,000	5,680,000
Income before income taxes	60,927,000	32,032,000	170,179,000	126,375,000
Income tax expense	15,437,000	6,760,000	48,741,000	28,800,000
Net income	45,490,000	25,272,000	121,438,000	97,575,000
Net income attributable to:
Stockholders	45,387,000	25,266,000	121,239,000	97,466,000
Non-controlling interests	103,000	6,000	199,000	109,000
Net income	$ 45,490,000	$ 25,272,000	$ 121,438,000	$ 97,575,000
Earnings per share attributable to stockholders:
Basic	$ 0.42	$ 0.23	$ 1.11	$ 0.90
Diluted	$ 0.41	$ 0.23	$ 1.10	$ 0.89
Weighted average number of shares outstanding:
Basic	109,018,469	108,003,390	108,887,026	108,453,525
Diluted	110,369,718	109,381,173	110,060,712	109,634,195
Service Revenue [Member]
Revenues:
Total revenue	$ 222,679,000	$ 178,577,000	$ 639,941,000	$ 585,555,000
Operating expenses:
Direct expenses	39,223,000	36,382,000	118,026,000	122,719,000
Inventory Sales Revenue [Member]
Revenues:
Total revenue	108,863,000	111,219,000	353,906,000	400,892,000
Operating expenses:
Direct expenses	$ 96,253,000	$ 102,410,000	$ 320,972,000	$ 372,703,000