UNAUDITED CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Revenues:
Total revenues	$ 926,458	$ 710,000	$ 1,734,606	$ 1,568,563
Cost of revenues: (exclusive of items shown separately below)
Total cost of revenues	617,886	470,681	1,178,422	1,077,347
Selling, general and administrative expenses	124,106	103,839	245,747	233,146
Accretion of environmental liabilities	2,873	2,766	5,826	5,327
Depreciation and amortization	71,592	72,494	143,755	147,027
Income from operations	110,001	60,220	160,856	105,716
Other expense, net	(1,480)	(500)	(2,708)	(2,865)
Loss on sale of businesses	0	(184)	0	(3,258)
Interest expense, net of interest income of $580, $668, $1,060 and $1,666, respectively	(18,051)	(18,654)	(35,969)	(37,441)
Income before provision for income taxes	90,470	40,882	122,179	62,152
Provision for income taxes	23,395	11,859	33,368	21,557
Net income	$ 67,075	$ 29,023	$ 88,811	$ 40,595
Earnings Per Share [Abstract]
Basic (in dollars per share)	$ 1.23	$ 0.52	$ 1.63	$ 0.73
Diluted (in dollars per share)	$ 1.22	$ 0.52	$ 1.62	$ 0.73
Shares used to compute earnings per share - Basic (in shares)	54,529	55,590	54,625	55,673
Shares used to compute earnings per share - Diluted (in shares)	54,854	55,748	54,945	55,882
Service revenues
Revenues:
Total revenues	$ 734,563	$ 637,839	$ 1,397,271	$ 1,357,706
Cost of revenues: (exclusive of items shown separately below)
Total cost of revenues	492,662	411,065	943,000	903,781
Product revenues
Revenues:
Total revenues	191,895	72,161	337,335	210,857
Cost of revenues: (exclusive of items shown separately below)
Total cost of revenues	$ 125,224	$ 59,616	$ 235,422	$ 173,566