CONSOLIDATED STATEMENTS OF INCOME (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2010		Sep. 30, 2010		Jun. 30, 2010		Mar. 31, 2010		Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Revenues	$ 545,886		$ 556,053		$ 447,235		$ 434,962		$ 417,058		$ 487,651		$ 471,639		$ 354,896		$ 1,984,136	$ 1,731,244	$ 1,074,220
Cost of revenues (exclusive of items shown separately below)	373,142	[1]	386,518	[1]	307,754	[1]	312,577	[1]	290,770	[1]	335,273	[1]	324,280	[1]	260,417	[1]	1,379,991	1,210,740	753,483
Selling, general and administrative expenses																	254,137	205,812	163,157
Accretion of environmental liabilities																	9,680	10,307	10,617
Depreciation and amortization																	122,663	92,473	64,898
Income from operations	59,247		66,792		51,884		39,742		42,998		73,372		71,923		23,619		217,665	211,912	82,065
Other income (expense)	471		164		2,868		2,899		310		(669)		2,708		446		6,402	2,795	259
Loss on early extinguishment of debt																	0	(2,294)	(4,853)
Interest expense, net of interest income of $798, 874, and $825, respectively																	(39,389)	(27,936)	(15,999)
Income from continuing operations, before provision for income taxes																	184,678	184,477	61,472
Provision for income taxes																	57,426	56,756	26,225
Income from continuing operations																	127,252	127,721	35,247
Income from discontinued operations, net of tax									0		0		2,412		382		0	2,794	1,439
Net income	$ 38,233		$ 37,133		$ 29,156		$ 22,730		$ 23,329		$ 38,827		$ 57,929		$ 10,430		$ 127,252	$ 130,515	$ 36,686
Earnings per share:
Basic (in dollars per share)	$ 0.72		$ 0.70		$ 0.55		$ 0.43		$ 0.44		$ 0.74		$ 1.1		$ 0.2		$ 2.40	$ 2.48	$ 0.74
Diluted (in dollars per share)	$ 0.72		$ 0.70		$ 0.55		$ 0.43		$ 0.44		$ 0.73		$ 1.1		$ 0.2		$ 2.39	$ 2.47	$ 0.74
Weighted average common shares outstanding (in shares)																	52,961	52,622	49,633
Weighted average common shares outstanding plus potentially dilutive common shares (in shares)																	53,324	52,932	49,866

[1] Items shown separately on the statements of income consist of (i) accretion of environmental liabilities and (ii) depreciation and amortization.