CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Income Statement [Abstract]
Revenues	$ 491,121	$ 472,418	$ 1,374,508	$ 1,475,081
Cost of revenues	303,707	271,401	859,512	826,479
Selling, general and administrative expenses	47,096	43,016	139,621	131,058
Depreciation and amortization	53,211	54,082	158,799	159,602
Loss on disposition of assets, net	1,268	1,551	5,665	4,859
Operating profit	85,839	102,368	210,911	353,083
Interest expense	(283)	(441)	(1,565)	(1,687)
Interest income	8	16	73	25
Other income, net	1,279	1,104	1,643	1,144
Income before income taxes	86,843	103,047	211,062	352,565
Income tax provision	33,083	37,007	81,810	133,510
Net income	$ 53,760	$ 66,040	$ 129,252	$ 219,055
Earnings per share
Basic (in dollars per share)	$ 0.25	$ 0.31	$ 0.60	$ 1.02
Diluted (in dollars per share)	$ 0.25	$ 0.30	$ 0.60	$ 1.01
Dividends per share (in dollars per share)	$ 0.10	$ 0.08	$ 0.30	$ 0.24
Weighted Average shares outstanding
Basic (in shares)	215,068	215,151	215,715	215,211
Diluted (in shares)	216,142	216,645	216,862	216,819