Segment Reporting (Summary of Financial Information Concerning Company's Reportable Segments) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Segment Reporting Information [Line Items]
Revenue	$ 524,693		$ 489,381		$ 1,006,402		$ 939,272
EBITDA	181,217	[1]	167,994	[1]	344,219	[1]	312,671	[1]
Intercompany Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	(66,435)	[2]	(66,739)	[2]	(125,385)	[2]	(123,607)	[2]
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue	591,128		556,120		1,131,787		1,062,879
EBITDA	183,565		168,256		350,516		315,538
Western [Member]
Segment Reporting Information [Line Items]
Revenue	206,269		202,833		401,624		395,984
EBITDA	63,262	[1]	62,386	[1]	125,754	[1]	120,962	[1]
Western [Member] | Intercompany Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	(24,635)	[2]	(26,677)	[2]	(46,882)	[2]	(49,872)	[2]
Western [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue	230,904		229,510		448,506		445,856
Central [Member]
Segment Reporting Information [Line Items]
Revenue	141,916		130,434		270,982		245,027
EBITDA	48,819	[1]	48,517	[1]	94,662	[1]	88,755	[1]
Central [Member] | Intercompany Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	(18,002)	[2]	(16,776)	[2]	(32,320)	[2]	(30,146)	[2]
Central [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue	159,918		147,210		303,302		275,173
Eastern [Member]
Segment Reporting Information [Line Items]
Revenue	100,058		95,725		191,384		183,494
EBITDA	30,768	[1]	29,029	[1]	57,904	[1]	54,909	[1]
Eastern [Member] | Intercompany Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	(20,492)	[2]	(19,817)	[2]	(38,533)	[2]	(37,477)	[2]
Eastern [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue	120,550		115,542		229,917		220,971
Exploration and Production [Member]
Segment Reporting Information [Line Items]
Revenue	76,450		60,389		142,412		114,767
EBITDA	40,716	[1]	28,324	[1]	72,196	[1]	50,912	[1]
Exploration and Production [Member] | Intercompany Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	(3,306)	[2]	(3,469)	[2]	(7,650)	[2]	(6,112)	[2]
Exploration and Production [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue	79,756		63,858		150,062		120,879
Corporate [Member]
Segment Reporting Information [Line Items]
EBITDA	$ (2,348)	[1],[3]	$ (262)	[1],[3]	$ (6,297)	[1],[3]	$ (2,867)	[1],[3]

[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] 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.