Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Cash flows from operating activities:
Net income	$ 66,112	$ 36,062
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	79,264	53,089
Amortization of intangible assets	11,247	7,391
Non-cash interest expense, net	783	725
Non-cash stock-based compensation expense	5,528	4,049
Benefit from deferred income taxes	(19,838)	(12,614)
Equity in earnings of unconsolidated affiliates	(7,346)	(7,834)
Gains on sales of assets, net	(1,965)	(2,407)
Other non-cash items, net	(5,578)	734
Changes in assets and liabilities, net of acquisitions:
Accounts receivable	(61,993)	48,337
Contract assets	26,799	26,311
Inventories	651	5,932
Other assets, current and long-term portion	(7,961)	17,906
Accounts payable and accrued expenses	153,386	34,058
Contract liabilities	14,782	2,603
Other liabilities, current and long-term portion	3,293	(11,076)
Net cash provided by operating activities	257,164	203,266
Cash flows from investing activities:
Cash paid for acquisitions, net of cash acquired	(88,646)	0
Capital expenditures	(48,058)	(60,594)
Proceeds from sale of property and equipment	6,035	8,363
Payments for other investments	(4,350)	(12,000)
Proceeds from other investments	557	648
Other investing activities, net	(150)	4,843
Net cash used in investing activities	(134,612)	(58,740)
Cash flows from financing activities:
Proceeds from credit facilities	18,155	675,935
Repayments of credit facilities	(8,869)	(671,780)
Payments of finance lease obligations	(38,222)	(30,856)
Proceeds from stock-based awards	0	1,476
Payments for stock-based awards	(3,753)	(572)
Repurchases of common stock	0	(119,427)
Other financing activities, net	(502)	0
Net cash used in financing activities	(33,191)	(145,224)
Effect of currency translation on cash	(72)	934
Net increase in cash and cash equivalents	89,289	236
Cash and cash equivalents - beginning of period	423,118	71,427
Cash and cash equivalents - end of period	512,407	71,663
Supplemental cash flow information:
Interest paid	21,689	21,479
Supplemental disclosure of non-cash information:
Additions to property and equipment from finance leases	$ 50,772	$ 26,932