CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues:
Total revenues	$ 858,204	$ 843,181	$ 849,140	$ 749,778	$ 747,403	$ 755,846	$ 752,788	$ 688,941	$ 3,300,303	$ 2,944,978	$ 2,755,226
Cost of revenues: (exclusive of items shown separately below)
Total cost of revenues	594,857	580,685	583,584	546,425	526,690	519,595	519,803	496,585	2,305,551	2,062,673	1,932,857
Selling, general and administrative expenses									503,747	456,648	422,015
Accretion of environmental liabilities									9,806	9,460	10,177
Depreciation and amortization									298,625	288,422	287,002
Goodwill impairment charge									0	0	34,013
(Loss) income from operations	41,485	65,745	64,353	10,991	27,935	47,663	46,744	5,433	182,574	127,775	69,162
Other (expense) income, net	(4,061)	(996)	846	(299)	(3,305)	(432)	(833)	(1,549)	(4,510)	(6,119)	6,195
Loss on early extinguishment of debt		(2,500)				(1,900)	(6,000)		(2,488)	(7,891)	0
Gain on sale of businesses							31,700		0	30,732	16,884
Interest expense, net of interest income of $2,958, $1,897, and $784, respectively									(81,094)	(85,808)	(83,525)
(Loss) income before (benefit) provision for income taxes									94,482	58,689	8,716
Provision (benefit) for income taxes									28,846	(42,050)	48,589
Net income (loss)	$ 16,431	$ 31,089	$ 30,747	$ (12,631)	$ 84,194	$ 12,058	$ 25,880	$ (21,393)	$ 65,636	$ 100,739	$ (39,873)
Earnings (loss) per share:
Basic (in dollars per share)	$ 0.29	$ 0.55	$ 0.55	$ (0.22)	$ 1.48	$ 0.21	$ 0.45	$ (0.37)	$ 1.17	$ 1.77	$ (0.69)
Diluted (in dollars per share)	$ 0.29	$ 0.55	$ 0.54	$ (0.22)	$ 1.48	$ 0.21	$ 0.45	$ (0.37)	$ 1.16	$ 1.76	$ (0.69)
Shares used to compute earnings (loss) per share — Basic (in shares)									56,148	57,072	57,532
Shares used to compute earnings (loss) per share — Diluted (in shares)									56,340	57,200	57,532
Service revenues
Revenues:
Total revenues									$ 2,709,239	$ 2,398,650	$ 2,280,809
Cost of revenues: (exclusive of items shown separately below)
Total cost of revenues									1,861,975	1,641,798	1,543,210
Product revenues
Revenues:
Total revenues									591,064	546,328	474,417
Cost of revenues: (exclusive of items shown separately below)
Total cost of revenues									$ 443,576	$ 420,875	$ 389,647