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, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Segment Reporting Information [Line Items]
Revenue	$ 546,551		$ 503,646		$ 1,552,953		$ 1,442,918
EBITDA	190,711	[1]	175,969	[1]	534,930	[1]	488,640	[1]
Intercompany Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	(68,157)	[2]	(65,704)	[2]	(193,543)	[2]	(189,311)	[2]
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue	614,708		569,350		1,746,496		1,632,229
EBITDA	191,863		175,781		542,379		491,319
Western [Member]
Segment Reporting Information [Line Items]
Revenue	215,849		210,211		617,473		606,195
EBITDA	69,140	[1]	67,773	[1]	194,894	[1]	188,736	[1]
Western [Member] | Intercompany Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	(24,895)	[2]	(26,638)	[2]	(71,778)	[2]	(76,510)	[2]
Western [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue	240,744		236,849		689,251		682,705
Central [Member]
Segment Reporting Information [Line Items]
Revenue	149,213		134,333		420,195		379,360
EBITDA	53,809	[1]	48,845	[1]	148,471	[1]	137,600	[1]
Central [Member] | Intercompany Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	(18,732)	[2]	(16,848)	[2]	(51,052)	[2]	(46,993)	[2]
Central [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue	167,945		151,181		471,247		426,353
Eastern [Member]
Segment Reporting Information [Line Items]
Revenue	102,624		96,060		294,008		279,554
EBITDA	29,379	[1]	28,038	[1]	87,284	[1]	82,947	[1]
Eastern [Member] | Intercompany Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	(21,010)	[2]	(19,884)	[2]	(59,543)	[2]	(57,362)	[2]
Eastern [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue	123,634		115,944		353,551		336,916
Exploration and Production [Member]
Segment Reporting Information [Line Items]
Revenue	78,865		63,042		221,277		177,809
EBITDA	39,535	[1]	31,125	[1]	111,730	[1]	82,036	[1]
Exploration and Production [Member] | Intercompany Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	(3,520)	[2]	(2,334)	[2]	(11,170)	[2]	(8,446)	[2]
Exploration and Production [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue	82,385		65,376		232,447		186,255
Corporate [Member]
Segment Reporting Information [Line Items]
EBITDA	$ (1,152)	[1],[3]	$ 188	[1],[3]	$ (7,449)	[1],[3]	$ (2,679)	[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.