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	$ 322,699	$ 394,096	$ 259,231
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	258,841	212,485	192,332
Amortization of intangible assets	38,910	22,997	20,598
Goodwill and intangible asset impairment	0	3,319	47,662
Non-cash interest expense, net	2,988	3,219	2,584
Non-cash stock-based compensation expense	21,875	16,447	13,527
Provision for deferred income taxes	7,180	22,160	56,209
Equity in earnings of unconsolidated affiliates	(29,738)	(27,367)	(23,855)
Gains on sales of assets, net	(16,210)	(13,908)	(16,052)
Other non-cash items, net	21,775	(2,768)	8,910
Changes in assets and liabilities, net of acquisitions:
Accounts receivable	87,372	(124,739)	(8,363)
Contract assets	63,306	237,800	(326,838)
Inventories	17,904	24,051	(29,366)
Other assets, current and long-term portion	20,486	10,180	28,709
Accounts payable and accrued expenses	94,069	(228,142)	251,735
Contract liabilities	21,326	(52,215)	28,411
Other liabilities, current and long-term portion	4,471	52,663	24,522
Net cash provided by operating activities	937,254	550,278	529,956
Cash flows from investing activities:
Cash paid for acquisitions, net of cash acquired	(24,971)	(179,481)	(6,684)
Capital expenditures	(213,746)	(126,473)	(180,420)
Proceeds from sale of property and equipment	37,077	35,015	39,359
Payments for other investments	(17,456)	(5,589)	(39,469)
Proceeds from other investments	648	14,705	5,415
Other investing activities, net	1,843	0	0
Net cash used in investing activities	(216,605)	(261,823)	(181,799)
Cash flows from financing activities:
Proceeds from credit facilities	1,434,610	3,025,927	3,418,232
Repayments of credit facilities	1,741,067	3,126,595	3,359,521
Proceeds from issuance of 4.50% senior notes	600,000	0	0
Repayments of 4.875% senior notes	(400,000)	0	0
Repayments of other borrowings, net	(62)	(12,438)	(17,427)
Payments of finance lease obligations	(126,988)	(88,341)	(72,167)
Payments of acquisition-related contingent consideration	10,097	34,267	15,929
(Distributions to) proceeds from non-controlling interests	(719)	583	(559)
Proceeds from stock-based awards	7,090	4,655	4,047
Payments for stock-based awards	(636)	(45)	(3,821)
Repurchases of common stock	(120,228)	(5,652)	(313,949)
Other financing activities, net	(11,790)	(8,458)	0
Net cash used in financing activities	(369,887)	(244,631)	(361,094)
Effect of currency translation on cash	929	181	33
Net increase (decrease) in cash and cash equivalents	351,691	44,005	(12,904)
Cash and cash equivalents - beginning of period	71,427	27,422	40,326
Cash and cash equivalents - end of period	423,118	71,427	27,422
Supplemental cash flow information:
Interest paid	65,016	84,971	81,230
Income tax payments (refunds), net	64,651	106,248	(21,450)
Supplemental disclosure of non-cash information:
Additions to property and equipment from finance leases	$ 114,221	$ 206,156	$ 75,545
4.50% Senior Notes [Member] | Senior Notes [Member]
Debt instrument, interest rate (percentage)	4.50%
4.875% Senior Notes [Member] | Senior Notes [Member]
Debt instrument, interest rate (percentage)	4.875%