CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Income Statement [Abstract]
Revenues	$ 457,566	$ 500,106	$ 883,387	$ 1,002,663
Cost of revenues	287,578	281,279	555,805	555,078
Selling, general and administrative expenses	47,611	43,115	92,525	88,042
Depreciation and amortization	52,767	53,950	105,588	105,520
Loss on disposition of assets, net	1,757	1,904	4,397	3,308
Operating profit	67,853	119,858	125,072	250,715
Interest expense	(942)	(650)	(1,282)	(1,246)
Interest income	60	4	65	9
Other (expense) income, net	(191)	(880)	364	40
Income before income taxes	66,780	118,332	124,219	249,518
Income tax provision	26,364	46,072	48,727	96,503
Net income	$ 40,416	$ 72,260	$ 75,492	$ 153,015
Earnings per share
Basic (in dollars per share)	$ 0.19	$ 0.34	$ 0.35	$ 0.71
Diluted (in dollars per share)	$ 0.19	$ 0.33	$ 0.35	$ 0.71
Dividends per share (in dollars per share)	$ 0.10	$ 0.08	$ 0.20	$ 0.16
Weighted Average shares outstanding
Basic (in shares)	215,883	214,893	216,039	215,241
Diluted (in shares)	216,695	216,127	217,190	216,780