Condensed Consolidated Income Statements - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Revenue:
Total revenue	$ 329,682,000	$ 331,542,000	$ 1,057,598,000	$ 993,847,000
Operating expenses:
Selling, general and administrative expenses	108,578,000	110,186,000	336,475,000	309,203,000
Acquisition-related costs	10,255,000		16,226,000
Depreciation and amortization expenses	21,907,000	18,436,000	64,912,000	55,586,000
Gain on disposition of property, plant and equipment	(1,068,000)	(276,000)	(1,311,000)	(1,536,000)
Foreign exchange loss	360,000	336,000	788,000	1,330,000
Total operating expenses	276,063,000	264,158,000	869,960,000	803,581,000
Operating income	53,619,000	67,384,000	187,638,000	190,266,000
Interest expense	(8,807,000)	(8,737,000)	(26,620,000)	(26,801,000)
Other income, net	602,000	2,280,000	2,800,000	6,714,000
Income before income taxes	45,414,000	60,927,000	163,818,000	170,179,000
Income tax expense	13,057,000	15,437,000	42,541,000	48,741,000
Net income	32,357,000	45,490,000	121,277,000	121,438,000
Net income attributable to:
Stockholders	32,336,000	45,387,000	121,273,000	121,239,000
Non-controlling interests	21,000	103,000	4,000	199,000
Net income	$ 32,357,000	$ 45,490,000	$ 121,277,000	$ 121,438,000
Earnings per share attributable to stockholders:
Basic	$ 0.29	$ 0.42	$ 1.10	$ 1.11
Diluted	$ 0.29	$ 0.41	$ 1.09	$ 1.10
Weighted average number of shares outstanding:
Basic	110,410,172	109,018,469	110,233,851	108,887,026
Diluted	111,391,396	110,369,718	111,333,247	110,060,712
Service Revenue [Member]
Revenue:
Total revenue	$ 214,193,000	$ 222,679,000	$ 672,971,000	$ 639,941,000
Operating expenses:
Direct expenses	33,038,000	39,223,000	108,107,000	118,026,000
Inventory Sales Revenue [Member]
Revenue:
Total revenue	115,489,000	108,863,000	384,627,000	353,906,000
Operating expenses:
Direct expenses	$ 102,993,000	$ 96,253,000	$ 344,763,000	$ 320,972,000