Segment Reporting (Summary of Financial Information Concerning Company's Reportable Segments) (Detail) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) segment	Sep. 30, 2014 USD ($)
Segment Reporting Information [Line Items]
Revenue		$ 547,938	$ 546,551	$ 1,585,350	$ 1,552,953
Segment EBITDA	[1]	187,229	190,711	$ 530,672	534,930
Number of operating segments | segment				4
Intercompany Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	[2]	(71,246)	(68,157)	$ (202,169)	(193,543)
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue		619,184	614,708	1,787,519	1,746,496
Segment EBITDA		184,755	191,863	525,816	542,379
Western [Member]
Segment Reporting Information [Line Items]
Revenue		230,454	215,849	658,468	617,473
Segment EBITDA	[1]	76,279	69,140	218,185	194,894
Western [Member] | Intercompany Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	[2]	(27,026)	(24,895)	(76,261)	(71,778)
Western [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue		257,480	240,744	734,729	689,251
Central [Member]
Segment Reporting Information [Line Items]
Revenue		156,939	149,213	441,686	420,195
Segment EBITDA	[1]	56,783	53,809	155,855	148,471
Central [Member] | Intercompany Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	[2]	(18,974)	(18,732)	(53,087)	(51,052)
Central [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue		175,913	167,945	494,773	471,247
Eastern [Member]
Segment Reporting Information [Line Items]
Revenue		109,030	102,624	314,268	294,008
Segment EBITDA	[1]	34,301	29,379	97,205	87,284
Eastern [Member] | Intercompany Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	[2]	(22,238)	(21,010)	(63,357)	(59,543)
Eastern [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue		131,268	123,634	377,625	353,551
Exploration and Production [Member]
Segment Reporting Information [Line Items]
Revenue		51,515	78,865	170,928	221,277
Segment EBITDA	[1]	17,392	39,535	54,571	111,730
Exploration and Production [Member] | Intercompany Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	[2]	(3,008)	(3,520)	(9,464)	(11,170)
Exploration and Production [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue		54,523	82,385	180,392	232,447
Corporate [Member]
Segment Reporting Information [Line Items]
Segment EBITDA	[1],[3]	$ 2,474	$ (1,152)	$ 4,856	$ (7,449)

[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.