UNAUDITED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Cash flows (used in) from operating activities:
Net income	$ 45,314	$ 21,736
Adjustments to reconcile net income to net cash (used in) from operating activities:
Depreciation and amortization	84,298	72,163
Allowance for doubtful accounts	3,619	2,446
Amortization of deferred financing costs and debt discount	1,561	900
Accretion of environmental liabilities	3,156	2,953
Changes in environmental liability estimates	312	275
Deferred income taxes	2,226	(39)
Other (income) expense, net	(704)	1,228
Stock-based compensation	5,712	3,480
Environmental expenditures	(3,615)	(3,011)
Changes in assets and liabilities, net of acquisitions:
Accounts receivable and unbilled accounts receivable	(138,690)	(9,703)
Inventories and supplies	(13,610)	(747)
Other current and non-current assets	(32,924)	(9,956)
Accounts payable	43,001	22,179
Other current and long-term liabilities	(38,285)	(904)
Net cash (used in) from operating activities	(38,629)	103,000
Cash flows used in investing activities:
Additions to property, plant and equipment	(70,308)	(41,913)
Proceeds from sale and disposal of fixed assets	1,320	1,204
Acquisitions, net of cash acquired	5,000	(22,918)
Additions to intangible assets including costs to obtain or renew permits	(321)	(505)
Proceeds from sale of available-for-sale securities	10,450	20,375
Purchases of available-for-sale securities	(5,002)	(42,980)
Net cash used in investing activities	(58,861)	(86,737)
Cash flows used in financing activities:
Change in uncashed checks	(2,295)	(6,662)
Tax payments related to withholdings on vested restricted stock	(1,831)	(3,719)
Repurchases of common stock	(3,694)	(26,546)
Deferred financing costs paid	(291)	(137)
Payments on finance leases	(3,585)	(1,672)
Principal payments on debt	(4,384)	(1,884)
Net cash used in financing activities	(16,080)	(40,620)
Effect of exchange rate change on cash	579	1,639
Decrease in cash and cash equivalents	(112,991)	(22,718)
Cash and cash equivalents, beginning of period	452,575	519,101
Cash and cash equivalents, end of period	339,584	496,383
Cash payments for interest and income taxes:
Interest paid	33,697	27,507
Income taxes paid, net of refunds	3,121	3,599
Non-cash investing activities:
Property, plant and equipment accrued	11,397	5,108
Remedial liability assumed in acquisition of property, plant and equipment	13,073	0
ROU assets obtained in exchange for operating lease liabilities	7,342	2,305
ROU assets obtained in exchange for finance lease liabilities	$ 4,679	$ 9,205