CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Cash flows from operating activities:
Net income	$ 203,247	$ 134,837	$ 97,740
Adjustments to reconcile net income to net cash from operating activities:
Depreciation and amortization	298,135	292,915	300,725
Allowance for doubtful accounts	8,018	10,133	2,408
Amortization of deferred financing costs and debt discount	4,245	3,666	3,809
Accretion of environmental liabilities	11,745	11,051	10,136
Changes in environmental liability estimates	2,979	10,698	(332)
Deferred income taxes	1,482	(9,748)	8,005
Other expense (income), net	515	290	(2,897)
Stock-based compensation	18,839	18,502	17,816
Loss (gain) on sale of businesses	0	3,376	(687)
Loss on early extinguishment of debt	0	0	6,131
Environmental expenditures	(15,506)	(12,401)	(18,701)
Changes in assets and liabilities, net of acquisitions:
Accounts receivable and unbilled accounts receivable	(96,551)	22,422	(33,271)
Inventories and supplies	(31,689)	(7,933)	(15,869)
Other current and non-current assets	9,268	(12,602)	(14,421)
Accounts payable	108,398	(80,328)	7,153
Other current and long-term liabilities	22,872	45,719	45,447
Net cash from operating activities	545,997	430,597	413,192
Cash flows used in investing activities:
Additions to property, plant and equipment	(241,856)	(196,256)	(216,324)
Proceeds from sale and disposal of fixed assets	22,156	9,623	11,655
Acquisitions, net of cash acquired	(1,253,232)	(8,839)	(29,363)
Additions to intangible assets including costs to obtain or renew permits	(3,848)	(2,029)	(3,904)
Purchases of available-for-sale securities	(129,234)	(70,891)	(35,836)
Proceeds from sale of available-for-sale securities	98,412	61,220	51,202
Proceeds from sale of businesses, net of transactional costs	0	7,712	4,714
Net cash used in investing activities	(1,507,602)	(199,460)	(217,856)
Cash flows from (used in) financing activities:
Change in uncashed checks	(1,806)	5,404	(3,705)
Tax payments related to withholdings on vested restricted stock	(10,805)	(5,331)	(7,429)
Repurchases of common stock	(54,410)	(74,844)	(21,390)
Deferred financing costs paid	(13,737)	(2,171)	(10,079)
Payments on finance leases	(8,458)	(4,469)	(586)
Premiums paid on early extinguishment of debt	0	0	(2,701)
Principal payments on debt	(7,535)	(7,535)	(852,535)
Proceeds from issuance of debt, net of discount	995,000	0	845,000
Borrowings from revolving credit facility	0	150,000	0
Payments on revolving credit facility	0	(150,000)	0
Net cash from (used in) financing activities	898,249	(88,946)	(53,425)
Effect of exchange rate change on cash	(3,170)	4,919	3,573
(Decrease) increase in cash and cash equivalents	(66,526)	147,110	145,484
Cash and cash equivalents, beginning of year	519,101	371,991	226,507
Cash and cash equivalents, end of year	452,575	519,101	371,991
Cash payments for interest and income taxes:
Interest paid	73,440	72,535	60,852
Income taxes paid, net of refunds	65,192	53,123	27,035
Non-cash investing activities:
Property, plant and equipment accrued	$ 19,264	$ 3,536	$ 30,964