Reconciliation of Primary Measure of Segment Profitability to Income Before Income Tax Provision (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
EBITDA	$ 144,678	[1]	$ 108,575	[1]
Depreciation	(51,649)		(37,173)
Amortization of intangibles	(6,438)		(5,631)
Gain (loss) on disposal of assets	322		(715)
Interest expense	(19,012)		(12,285)
Other income, net	742		819
Income before income tax provision	68,643		53,590
Western
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
EBITDA	58,576	[1]	53,802	[1]
Central
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
EBITDA	40,238	[1]	35,450	[1]
Eastern
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
EBITDA	25,881	[1]	24,534	[1]
Exploration and Production
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
EBITDA	22,587	[1]	1,932	[1]
Reportable Segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
EBITDA	147,282		115,718
Unallocated corporate overhead
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
EBITDA	$ (2,604)	[1],[2]	$ (7,143)	[1],[2]

[1] For those items included in the determination of EBITDA, the accounting policies of the segments are the same as those described in the Company's most recent Annual Report on Form 10-K.

[2] 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 four operating segments.