CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Statement [Abstract]
Revenues	$ 467,926	$ 398,810	$ 904,260	$ 696,929
Cost of revenues (exclusive of items shown below)	312,079	254,016	607,684	470,258
Selling, general and administrative expenses	42,534	40,288	86,348	77,445
Depreciation and amortization	40,094	41,263	77,574	85,926
Gain on disposition of assets, net	(1,810)	(3,759)	(3,173)	(5,276)
Operating income	75,029	67,002	135,827	68,576
Interest expense	(113)	(114)	(218)	(217)
Interest income	458	411	860	540
Other income, net	4,104	2,010	9,499	2,222
Income before income taxes	79,478	69,309	145,968	71,121
Income tax provision	19,535	25,469	33,895	23,647
Net income	$ 59,943	$ 43,840	$ 112,073	$ 47,474
Earnings per share
Basic (in dollars per share)	$ 0.28	$ 0.20	$ 0.52	$ 0.22
Diluted (in dollars per share)	0.28	$ 0.20	0.52	$ 0.22
Dividends per share (in dollars per share)	$ 0.10		$ 0.20