UNAUDITED CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Revenues:
Total revenues	$ 710,000	$ 868,678	$ 1,568,563	$ 1,649,517
Cost of revenues: (exclusive of items shown separately below)
Total cost of revenues	470,681	594,933	1,077,347	1,159,297
Selling, general and administrative expenses	103,839	123,920	233,146	238,732
Accretion of environmental liabilities	2,766	2,560	5,327	5,134
Depreciation and amortization	72,494	74,217	147,027	149,572
Income from operations	60,220	73,048	105,716	96,782
Other (expense) income, net	(500)	(564)	(2,865)	2,419
Loss on sale of businesses	(184)	0	(3,258)	0
Interest expense, net of interest income of $668, $903, $1,666 and $1,829, respectively	(18,654)	(20,215)	(37,441)	(39,979)
Income before provision for income taxes	40,882	52,269	62,152	59,222
Provision for income taxes	11,859	16,025	21,557	22,002
Net income	$ 29,023	$ 36,244	$ 40,595	$ 37,220
Earnings Per Share [Abstract]
Basic (in USD per share)	$ 0.52	$ 0.65	$ 0.73	$ 0.67
Diluted (in USD per share)	$ 0.52	$ 0.65	$ 0.73	$ 0.66
Shares used to compute earnings per share - Basic (in shares)	55,590	55,875	55,673	55,861
Shares used to compute earnings per share - Diluted (in shares)	55,748	56,066	55,882	56,001
Service revenues
Revenues:
Total revenues	$ 637,839	$ 715,085	$ 1,357,706	$ 1,371,743
Cost of revenues: (exclusive of items shown separately below)
Total cost of revenues	411,065	480,229	903,781	943,712
Product revenues
Revenues:
Total revenues	72,161	153,593	210,857	277,774
Cost of revenues: (exclusive of items shown separately below)
Total cost of revenues	$ 59,616	$ 114,704	$ 173,566	$ 215,585