Segment Reporting (Summary of Financial Information Concerning Company's Reportable Segments) (Detail) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) segment	Jun. 30, 2014 USD ($)
Segment Reporting Information [Line Items]
Revenue		$ 531,312	$ 524,693	$ 1,037,412	$ 1,006,402
Segment EBITDA	[1]	176,611	181,217	$ 343,443	344,219
Number of operating segments | segment				4
Intercompany Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	[2]	(69,459)	(66,435)	$ (130,923)	(125,385)
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue		600,771	591,128	1,168,335	1,131,787
Segment EBITDA		173,772	183,565	341,062	350,516
Western [Member]
Segment Reporting Information [Line Items]
Revenue		222,388	206,269	428,014	401,624
Segment EBITDA	[1]	73,015	63,262	141,907	125,754
Western [Member] | Intercompany Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	[2]	(25,795)	(24,635)	(49,234)	(46,882)
Western [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue		248,183	230,904	477,248	448,506
Central [Member]
Segment Reporting Information [Line Items]
Revenue		149,812	141,916	284,747	270,982
Segment EBITDA	[1]	51,722	48,819	99,072	94,662
Central [Member] | Intercompany Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	[2]	(18,503)	(18,002)	(34,114)	(32,320)
Central [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue		168,315	159,918	318,861	303,302
Eastern [Member]
Segment Reporting Information [Line Items]
Revenue		107,215	100,058	205,239	191,384
Segment EBITDA	[1]	32,832	30,768	62,904	57,904
Eastern [Member] | Intercompany Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	[2]	(22,043)	(20,492)	(41,118)	(38,533)
Eastern [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue		129,258	120,550	246,357	229,917
Exploration and Production [Member]
Segment Reporting Information [Line Items]
Revenue		51,897	76,450	119,412	142,412
Segment EBITDA	[1]	16,203	40,716	37,179	72,196
Exploration and Production [Member] | Intercompany Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	[2]	(3,118)	(3,306)	(6,457)	(7,650)
Exploration and Production [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue		55,015	79,756	125,869	150,062
Corporate [Member]
Segment Reporting Information [Line Items]
Segment EBITDA	[1],[3]	$ 2,839	$ (2,348)	$ 2,381	$ (6,297)

[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.