CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net income	$ 97,740	$ 65,636	$ 100,739
Adjustments to reconcile net income to net cash from operating activities:
Depreciation and amortization	300,725	298,625	288,422
Allowance for doubtful accounts	2,408	15,817	7,901
Amortization of deferred financing costs and debt discount	3,809	3,846	3,482
Accretion of environmental liabilities	10,136	9,806	9,460
Changes in environmental liability estimates	(332)	2,147	(195)
Deferred income taxes	8,005	19,089	(83,335)
Other (income) expense, net	(2,897)	4,510	6,119
Stock-based compensation	17,816	16,792	13,146
Gain on sale of businesses	(687)	0	(30,732)
Loss on early extinguishment of debt	6,131	2,488	7,891
Environmental expenditures	(18,701)	(10,115)	(12,965)
Changes in assets and liabilities, net of acquisitions:
Accounts receivable and unbilled accounts receivable	(33,271)	(79,563)	(33,764)
Inventories and supplies	(15,869)	(26,958)	(5,002)
Other current assets	(14,421)	(7,946)	16,720
Accounts payable	7,153	46,915	(10,684)
Other current and long-term liabilities	45,447	12,121	8,495
Net cash from operating activities	413,192	373,210	285,698
Cash flows used in investing activities:
Additions to property, plant and equipment	(216,324)	(193,344)	(167,007)
Proceeds from sale and disposal of fixed assets	11,655	15,445	7,124
Acquisitions, net of cash acquired	(29,363)	(151,023)	(49,227)
Additions to intangible assets including costs to obtain or renew permits	(3,904)	(4,688)	(1,617)
Purchases of available-for-sale securities	(35,836)	(44,772)	(38,342)
Proceeds from sale of available-for-sale securities	51,202	28,723	376
Proceeds from sale of businesses, net of transactional costs	4,714	0	45,426
Net cash used in investing activities	(217,856)	(349,659)	(203,267)
Cash flows used in financing activities:
Change in uncashed checks	(3,705)	132	(5,940)
Proceeds from exercise of stock options	0	0	46
Tax payments related to withholdings on vested restricted stock	(7,429)	(3,266)	(3,149)
Repurchases of common stock	(21,390)	(45,080)	(48,971)
Deferred financing costs paid	(10,079)	(4,027)	(5,718)
Payments on finance leases	(586)	0	0
Premiums paid on early extinguishment of debt	(2,701)	(1,238)	(6,028)
Principal payments on debt	(852,535)	(405,768)	(402,000)
Proceeds from issuance of debt, net of discount	845,000	348,250	399,000
Borrowing from revolving credit facility	0	50,000	0
Payment on revolving credit facility	0	(50,000)	0
Net cash used in financing activities	(53,425)	(110,997)	(72,760)
Effect of exchange rate change on cash	3,573	(5,446)	2,731
Increase (decrease) in cash and cash equivalents	145,484	(92,892)	12,402
Cash and cash equivalents, beginning of year	226,507	319,399	306,997
Cash and cash equivalents, end of year	371,991	226,507	319,399
Cash payments for interest and income taxes:
Interest paid	60,852	89,171	93,174
Income taxes paid	27,035	20,036	18,682
Non-cash investing activities:
Property, plant and equipment accrued	30,964	15,657	16,109
Transfer of inventory to property, plant and equipment	0	0	12,641
Payable for estimated purchase price adjustment	$ 0	$ 4,032	$ 0