Segment Reporting (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Summary of financial information concerning the Company's reportable segments
Gross Revenues	$ 431,656	$ 379,141
Less: Intercompany elimination	(55,226)	(47,673)
Net Revenues	376,430	331,468
Operating income before depreciation, amortization and gain (loss) on disposal of assets	108,575	105,564
Western [Member]
Summary of financial information concerning the Company's reportable segments
Gross Revenues	204,681	197,466
Less: Intercompany elimination	(24,021)	(22,901)
Net Revenues	180,660	174,565
Operating income before depreciation, amortization and gain (loss) on disposal of assets	53,802	54,453
Central [Member]
Summary of financial information concerning the Company's reportable segments
Gross Revenues	119,817	111,959
Less: Intercompany elimination	(13,084)	(11,562)
Net Revenues	106,733	100,397
Operating income before depreciation, amortization and gain (loss) on disposal of assets	37,382	35,424
Eastern [Member]
Summary of financial information concerning the Company's reportable segments
Gross Revenues	107,158	69,716
Less: Intercompany elimination	(18,121)	(13,210)
Net Revenues	89,037	56,506
Operating income before depreciation, amortization and gain (loss) on disposal of assets	24,534	16,964
Corporate [Member]
Summary of financial information concerning the Company's reportable segments
Operating income before depreciation, amortization and gain (loss) on disposal of assets	$ (7,143)	$ (1,277)