Segment Reporting (Summary of Financial Information Concerning Company's Reportable Segments) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015 segment		Mar. 31, 2014
Segment Reporting Information [Line Items]
Revenue	$ 506,100		$ 481,710
Segment EBITDA	166,833	[1]	163,002	[1]
Number of operating segments	4
Intercompany Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	(61,464)	[2]	(58,950)	[2]
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue	567,564		540,660
Segment EBITDA	167,290		166,951
Western [Member]
Segment Reporting Information [Line Items]
Revenue	205,626		195,355
Segment EBITDA	68,892	[1]	62,492	[1]
Western [Member] | Intercompany Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	(23,439)	[2]	(22,248)	[2]
Western [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue	229,065		217,603
Central [Member]
Segment Reporting Information [Line Items]
Revenue	134,935		129,067
Segment EBITDA	47,350	[1]	45,843	[1]
Central [Member] | Intercompany Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	(15,611)	[2]	(14,317)	[2]
Central [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue	150,546		143,384
Eastern [Member]
Segment Reporting Information [Line Items]
Revenue	98,024		91,326
Segment EBITDA	30,072	[1]	27,137	[1]
Eastern [Member] | Intercompany Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	(19,075)	[2]	(18,041)	[2]
Eastern [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue	117,099		109,367
Exploration and Production [Member]
Segment Reporting Information [Line Items]
Revenue	67,515		65,962
Segment EBITDA	20,976	[1]	31,479	[1]
Exploration and Production [Member] | Intercompany Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	(3,339)	[2]	(4,344)	[2]
Exploration and Production [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue	70,854		70,306
Corporate [Member]
Segment Reporting Information [Line Items]
Segment EBITDA	$ (457)	[1],[3]	$ (3,949)	[1],[3]

[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] 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.

[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 four operating segments.