Segment Reporting (Reconciliation of Primary Measure of Segment Profitability to Income before Income Tax Provision) (Detail) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2015
Segment EBITDA	[1]	$ 159,806	$ 166,833
Depreciation		(60,897)	(57,307)
Amortization of intangibles		(7,694)	(6,999)
Other operating items		(236)	(662)
Interest expense		(17,184)	(15,697)
Other income (expense), net		222	(220)
Income before income tax provision		74,017	85,948
Western [Member]
Segment EBITDA	[1]	73,789	68,892
Central [Member]
Segment EBITDA	[1]	49,837	47,350
Eastern [Member]
Segment EBITDA	[1]	38,246	30,072
Exploration and Production [Member]
Segment EBITDA	[1]	6,422	20,976
Corporate [Member]
Segment EBITDA	[1],[2]	(8,488)	(457)
Operating Segments [Member]
Segment EBITDA		$ 168,294	$ 167,290

[1] For those items included in the determination of segment 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.