CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Income Statement [Abstract]
Revenues	$ 501,692	$ 425,821
Cost of revenues (exclusive of items shown below)	330,015	268,227
Selling, general and administrative expenses	48,708	44,914
Depreciation and amortization	55,505	52,821
Loss on disposition of assets, net	2,232	2,640
Operating profit	65,232	57,219
Interest expense	(337)	(340)
Interest income	4	5
Other income, net	80	555
Income before income taxes	64,979	57,439
Income tax provision	25,591	22,363
Net income	$ 39,388	$ 35,076
Earnings per share
Basic (in dollars per share)	$ 0.18	$ 0.16
Diluted (in dollars per share)	$ 0.18	$ 0.16
Dividends per share	$ 0.105	$ 0.100
Weighted average shares outstanding
Basic (in shares)	215,175	216,194
Diluted (in shares)	216,214	217,525