CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities:
Net income	$ 134,837	$ 97,740	$ 65,636
Adjustments to reconcile net income to net cash from operating activities:
Depreciation and amortization	292,915	300,725	298,625
Allowance for doubtful accounts	10,133	2,408	15,817
Amortization of deferred financing costs and debt discount	3,666	3,809	3,846
Accretion of environmental liabilities	11,051	10,136	9,806
Changes in environmental liability estimates	10,698	(332)	2,147
Deferred income taxes	(9,748)	8,005	19,089
Other expense (income), net	290	(2,897)	4,510
Stock-based compensation	18,502	17,816	16,792
Loss (gain) on sale of businesses	3,376	(687)	0
Loss on early extinguishment of debt	0	6,131	2,488
Environmental expenditures	(12,401)	(18,701)	(10,115)
Changes in assets and liabilities, net of acquisitions:
Accounts receivable and unbilled accounts receivable	22,422	(33,271)	(79,563)
Inventories and supplies	(7,933)	(15,869)	(26,958)
Other current and non-current assets	(12,602)	(14,421)	(7,946)
Accounts payable	(80,328)	7,153	46,915
Other current and long-term liabilities	45,719	45,447	12,121
Net cash from operating activities	430,597	413,192	373,210
Cash flows used in investing activities:
Additions to property, plant and equipment	(196,256)	(216,324)	(193,344)
Proceeds from sale and disposal of fixed assets	9,623	11,655	15,445
Acquisitions, net of cash acquired	(8,839)	(29,363)	(151,023)
Additions to intangible assets including costs to obtain or renew permits	(2,029)	(3,904)	(4,688)
Purchases of available-for-sale securities	(70,891)	(35,836)	(44,772)
Proceeds from sale of available-for-sale securities	61,220	51,202	28,723
Proceeds from sale of businesses, net of transactional costs	7,712	4,714	0
Net cash used in investing activities	(199,460)	(217,856)	(349,659)
Cash flows used in financing activities:
Change in uncashed checks	5,404	(3,705)	132
Tax payments related to withholdings on vested restricted stock	(5,331)	(7,429)	(3,266)
Repurchases of common stock	(74,844)	(21,390)	(45,080)
Deferred financing costs paid	(2,171)	(10,079)	(4,027)
Payments on finance leases	(4,469)	(586)	0
Premiums paid on early extinguishment of debt	0	(2,701)	(1,238)
Principal payments on debt	(7,535)	(852,535)	(405,768)
Proceeds from issuance of debt, net of discount	0	845,000	348,250
Borrowings from revolving credit facility	150,000	0	50,000
Payments on revolving credit facility	(150,000)	0	(50,000)
Net cash used in financing activities	(88,946)	(53,425)	(110,997)
Effect of exchange rate change on cash	4,919	3,573	(5,446)
Increase (decrease) in cash and cash equivalents	147,110	145,484	(92,892)
Cash and cash equivalents, beginning of year	371,991	226,507	319,399
Cash and cash equivalents, end of year	519,101	371,991	226,507
Cash payments for interest and income taxes:
Interest paid	72,535	60,852	89,171
Income taxes paid, net of refunds	53,123	27,035	20,036
Non-cash investing activities:
Property, plant and equipment accrued	3,536	30,964	15,657
Payable for estimated purchase price adjustment	$ 0	$ 0	$ 4,032