UNAUDITED CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Revenues:
Total revenues	$ 951,479	$ 779,344	$ 2,686,085	$ 2,347,907
Cost of revenues: (exclusive of items shown separately below)
Total cost of revenues	639,232	511,629	1,817,654	1,588,976
Selling, general and administrative expenses	133,164	106,544	378,911	339,690
Accretion of environmental liabilities	2,799	2,822	8,625	8,149
Depreciation and amortization	71,451	74,470	215,206	221,497
Income from operations	104,833	83,879	265,689	189,595
Other income (expense), net	199	2,268	(2,509)	(597)
Loss on sale of businesses	0	(118)	0	(3,376)
Interest expense, net of interest income of $653, $1,236, $1,712 and $2,902, respectively	(17,984)	(17,407)	(53,953)	(54,848)
Income before provision for income taxes	87,048	68,622	209,227	130,774
Provision for income taxes	21,605	13,712	54,973	35,269
Net income	$ 65,443	$ 54,910	$ 154,254	$ 95,505
Earnings Per Share [Abstract]
Basic (in dollars per share)	$ 1.20	$ 0.99	$ 2.83	$ 1.72
Diluted (in dollars per share)	$ 1.20	$ 0.99	$ 2.81	$ 1.71
Shares used to compute earnings per share - Basic (in shares)	54,411	55,592	54,553	55,646
Shares used to compute earnings per share - Diluted (in shares)	54,707	55,738	54,862	55,832
Service revenues
Revenues:
Total revenues	$ 744,549	$ 681,306	$ 2,141,820	$ 2,039,012
Cost of revenues: (exclusive of items shown separately below)
Total cost of revenues	508,498	428,735	1,451,498	1,332,516
Product revenues
Revenues:
Total revenues	206,930	98,038	544,265	308,895
Cost of revenues: (exclusive of items shown separately below)
Total cost of revenues	$ 130,734	$ 82,894	$ 366,156	$ 256,460