Condensed Consolidated Income Statements - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Revenues:
Total revenue	$ 389,050,000	$ 393,222,000	$ 662,305,000	$ 696,651,000
Operating expenses:
Selling, general and administrative expenses	100,632,000	97,714,000	199,017,000	192,898,000
Acquisition-related costs		38,000		707,000
Depreciation and amortization expenses	17,857,000	17,112,000	37,150,000	34,227,000
Gain on disposition of property, plant and equipment	(1,213,000)	(101,000)	(1,260,000)	(250,000)
Foreign exchange loss	392,000	403,000	994,000	881,000
Total operating expenses	300,250,000	315,252,000	539,423,000	585,093,000
Operating income	88,800,000	77,970,000	122,882,000	111,558,000
Interest expense	(8,882,000)	(10,117,000)	(18,064,000)	(20,933,000)
Other income, net	857,000	1,679,000	4,434,000	3,718,000
Income before income taxes	80,775,000	69,532,000	109,252,000	94,343,000
Income tax expense	27,656,000	15,401,000	33,304,000	22,040,000
Net income	53,119,000	54,131,000	75,948,000	72,303,000
Net income attributable to:
Stockholders	53,043,000	54,036,000	75,851,000	72,200,000
Non-controlling interests	76,000	95,000	97,000	103,000
Net income	$ 53,119,000	$ 54,131,000	$ 75,948,000	$ 72,303,000
Earnings per share attributable to stockholders:
Basic	$ 0.49	$ 0.50	$ 0.70	$ 0.66
Diluted	$ 0.49	$ 0.49	$ 0.69	$ 0.66
Weighted average number of shares outstanding:
Basic	108,387,490	108,707,708	108,818,903	108,725,871
Diluted	109,323,343	109,942,768	109,903,808	109,982,763
Service Revenue [Member]
Revenues:
Total revenue	$ 234,139,000	$ 234,606,000	$ 417,262,000	$ 406,978,000
Operating expenses:
Direct expenses	39,448,000	50,268,000	78,803,000	86,337,000
Inventory Sales Revenue [Member]
Revenues:
Total revenue	154,911,000	158,616,000	245,043,000	289,673,000
Operating expenses:
Direct expenses	$ 143,134,000	$ 149,818,000	$ 224,719,000	$ 270,293,000