Segment Reporting - Summary of Financial Information Concerning Company's Reportable Segments (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011
Segment Reporting Information [Line Items]
Gross Revenues	$ 484,599		$ 461,685		$ 1,386,995		$ 1,288,045
Less: Intercompany elimination	(58,945)	[1]	(57,723)	[1]	(174,180)	[1]	(162,431)	[1]
Net Revenues	425,654		403,962		1,212,815		1,125,614
Operating Income Before Depreciation, Amortization, Gain (Loss) on Disposal of Assets and Gain From Litigation Settlement	134,434	[2]	134,354	[2]	370,565	[2]	367,061	[2]
Corporate (Member)
Segment Reporting Information [Line Items]
Operating Income Before Depreciation, Amortization, Gain (Loss) on Disposal of Assets and Gain From Litigation Settlement	(1,992)	[2],[3]	1,695	[2],[3]	(9,823)	[2],[3]	3,351	[2],[3]
Western [Member]
Segment Reporting Information [Line Items]
Gross Revenues	228,906		223,156		660,679		633,784
Less: Intercompany elimination	(24,943)	[1]	(25,896)	[1]	(74,513)	[1]	(74,407)	[1]
Net Revenues	203,963		197,260		586,166		559,377
Operating Income Before Depreciation, Amortization, Gain (Loss) on Disposal of Assets and Gain From Litigation Settlement	61,119	[2]	65,306	[2]	173,810	[2]	177,593	[2]
Central [Member]
Segment Reporting Information [Line Items]
Gross Revenues	142,940		126,389		393,433		362,351
Less: Intercompany elimination	(14,007)	[1]	(13,538)	[1]	(42,215)	[1]	(38,589)	[1]
Net Revenues	128,933		112,851		351,218		323,762
Operating Income Before Depreciation, Amortization, Gain (Loss) on Disposal of Assets and Gain From Litigation Settlement	49,194	[2]	40,174	[2]	128,699	[2]	115,261	[2]
Eastern [Member]
Segment Reporting Information [Line Items]
Gross Revenues	112,753		112,140		332,883		291,910
Less: Intercompany elimination	(19,995)	[1]	(18,289)	[1]	(57,452)	[1]	(49,435)	[1]
Net Revenues	92,758		93,851		275,431		242,475
Operating Income Before Depreciation, Amortization, Gain (Loss) on Disposal of Assets and Gain From Litigation Settlement	$ 26,113	[2]	$ 27,179	[2]	$ 77,879	[2]	$ 70,856	[2]

[1] Intercompany revenues reflect each segment's total intercompany sales, including intercompany sales within a segment and between segments. Transactions within and between segments are generally made on a basis intended to reflect the market value of the service.

[2] For those items included in the determination of operating income before depreciation, amortization, gain (loss) on disposal of assets and gain from litigation settlement, the accounting policies of the segments are the same as those described in the Company's most recent Annual Report on Form 10-K.

[3] Corporate functions include accounting, legal, tax, treasury, information technology, risk management, human resources, training and other administrative functions. Amounts reflected are net of allocations to the three geographic operating segments.