CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Income Statement [Abstract]
Revenues	$ 582,831	$ 457,566	$ 1,084,523	$ 883,387
Cost of revenues (exclusive of items shown below)	374,275	287,578	704,290	555,805
Selling, general and administrative expenses	47,603	47,611	96,311	92,525
Depreciation and amortization	56,517	52,767	112,022	105,588
Loss on disposition of assets, net	1,405	1,757	3,637	4,397
Operating profit	103,031	67,853	168,263	125,072
Interest expense	(49)	(942)	(386)	(1,282)
Interest income	6	60	10	65
Other income (expense), net	831	(191)	911	364
Income before income taxes	103,819	66,780	168,798	124,219
Income tax provision	40,536	26,364	66,127	48,727
Net income	$ 63,283	$ 40,416	$ 102,671	$ 75,492
Earnings per share
Basic (in dollars per share)	$ 0.29	$ 0.19	$ 0.48	$ 0.35
Diluted (in dollars per share)	$ 0.29	$ 0.19	$ 0.47	$ 0.35
Dividends per share (in dollars per share)	$ 0.105	$ 0.10	$ 0.21	$ 0.20
Weighted average shares outstanding
Basic (in shares)	215,224	215,883	215,199	216,039
Diluted (in shares)	216,238	216,695	216,280	217,190