CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Revenues	$ 439,994	$ 470,999	$ 1,344,254	$ 1,167,928
Cost of revenues (exclusive of items shown below)	300,947	294,820	908,631	765,078
Selling, general and administrative expenses	41,787	39,738	128,135	117,183
Depreciation and amortization	42,993	39,587	120,567	125,513
Gain on disposition of assets, net	(286)	(503)	(3,459)	(5,779)
Operating income	54,553	97,357	190,380	165,933
Interest expense	(150)	(105)	(368)	(322)
Interest income	783	488	1,643	1,028
Other income, net	287	564	9,786	2,786
Income before income taxes	55,473	98,304	201,441	169,425
Income tax provision	5,506	40,970	39,401	64,617
Net income	$ 49,967	$ 57,334	$ 162,040	$ 104,808
Earnings per share
Basic (in dollars per share)	$ 0.23	$ 0.26	$ 0.75	$ 0.48
Diluted (in dollars per share)	0.23	0.26	0.75	0.48
Dividends per share (in dollars per share)	$ 0.10	$ 0.06	$ 0.30	$ 0.06