CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Income Statement [Abstract]
Revenues	$ 620,684	$ 491,121	$ 1,705,207	$ 1,374,508
Cost of revenues (exclusive of items shown below)	398,306	303,707	1,102,596	859,512
Selling, general and administrative expenses	50,814	47,096	147,125	139,621
Depreciation and amortization	57,219	53,211	169,241	158,799
Loss on disposition of assets, net	7,684	1,268	11,321	5,665
Operating profit	106,661	85,839	274,924	210,911
Interest expense	(456)	(283)	(842)	(1,565)
Interest income	4	8	14	73
Other (expense) income, net	(454)	1,279	457	1,643
Income before income taxes	105,755	86,843	274,553	211,062
Income tax provision	40,870	33,083	106,997	81,810
Net income	$ 64,885	$ 53,760	$ 167,556	$ 129,252
Earnings per share
Basic (in dollars per share)	$ 0.30	$ 0.25	$ 0.78	$ 0.60
Diluted (in dollars per share)	$ 0.30	$ 0.25	$ 0.77	$ 0.60
Dividends per share (in dollars per share)	$ 0.105	$ 0.100	$ 0.315	$ 0.300
Weighted average shares outstanding
Basic (in shares)	215,202	215,068	215,200	215,715
Diluted (in shares)	216,334	216,142	216,316	216,862