Segment Reporting (Summary of Financial Information Concerning Company's Reportable Segments) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended										12 Months Ended
	Dec. 31, 2014		Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013		Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Segment Reporting Information [Line Items]
Revenue	$ 526,213		$ 546,551	$ 524,693	$ 481,710	$ 485,877		$ 503,646	$ 489,381	$ 449,892	$ 2,079,166		$ 1,928,795		$ 1,661,618
Segment EBITDA											711,304	[1]	651,339	[1]	507,807	[1]
Depreciation and amortization											257,944		243,864		193,584
Capital expenditures											241,277		209,874		153,517
Total assets	5,250,031	[2]				5,064,252	[2]				5,250,031	[2]	5,064,252	[2]	5,076,026	[2]
Intercompany Revenues [Member]
Segment Reporting Information [Line Items]
Revenue											(259,352)	[3]	(249,830)	[3]	(232,759)	[3]
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue											2,338,518		2,178,625		1,894,377
Segment EBITDA											718,738		651,567		518,880
Western [Member]
Segment Reporting Information [Line Items]
Revenue											823,922		805,790		782,134
Segment EBITDA											258,126	[1]	249,548	[1]	229,427	[1]
Depreciation and amortization											79,907		81,164		78,191
Capital expenditures											65,227		70,960		64,583
Total assets	1,482,474	[2]				1,487,409	[2]				1,482,474	[2]	1,487,409	[2]	1,507,081	[2]
Western [Member] | Intercompany Revenues [Member]
Segment Reporting Information [Line Items]
Revenue											(96,194)	[3]	(99,974)	[3]	(100,094)	[3]
Western [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue											920,116		905,764		882,228
Segment EBITDA											258,126		249,548		229,427
Central [Member]
Segment Reporting Information [Line Items]
Revenue											561,480		510,928		461,875
Segment EBITDA											197,121	[1]	182,790	[1]	164,756	[1]
Depreciation and amortization											69,037		64,165		54,743
Capital expenditures											77,500		57,952		51,300
Total assets	1,187,505	[2]				1,173,089	[2]				1,187,505	[2]	1,173,089	[2]	1,114,479	[2]
Central [Member] | Intercompany Revenues [Member]
Segment Reporting Information [Line Items]
Revenue											(68,094)	[3]	(62,438)	[3]	(56,041)	[3]
Central [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue											629,574		573,366		517,916
Segment EBITDA											197,121		182,790		164,756
Eastern [Member]
Segment Reporting Information [Line Items]
Revenue											393,821		371,772		366,825
Segment EBITDA											116,230	[1]	108,173	[1]	101,046	[1]
Depreciation and amortization											53,717		51,546		49,289
Capital expenditures											60,384		39,703		32,037
Total assets	852,963	[2]				807,124	[2]				852,963	[2]	807,124	[2]	826,687	[2]
Eastern [Member] | Intercompany Revenues [Member]
Segment Reporting Information [Line Items]
Revenue											(80,162)	[3]	(76,072)	[3]	(75,082)	[3]
Eastern [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue											473,983		447,844		441,907
Segment EBITDA											116,230		108,173		101,046
Exploration and Production [Member]
Segment Reporting Information [Line Items]
Revenue											299,943		240,305		50,784	[4],[5]
Segment EBITDA											147,261	[1]	111,056	[1]	23,651	[1],[4],[5]
Depreciation and amortization											52,709		44,099		9,018	[4],[5]
Capital expenditures											36,608		34,916		1,971	[4],[5]
Total assets	1,609,553	[2]				1,484,501	[2]				1,609,553	[2]	1,484,501	[2]	1,493,693	[2],[4],[5]
Exploration and Production [Member] | Intercompany Revenues [Member]
Segment Reporting Information [Line Items]
Revenue											(14,902)	[3]	(11,346)	[3]	(1,542)	[3],[4],[5]
Exploration and Production [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue											314,845		251,651		52,326	[4],[5]
Segment EBITDA											147,261		111,056		23,651
Corporate [Member]
Segment Reporting Information [Line Items]
Segment EBITDA											(7,434)	[1],[4],[5]	(228)	[1],[4],[5]	(11,073)	[1],[4],[5]
Depreciation and amortization											2,574	[4],[5]	2,890	[4],[5]	2,343	[4],[5]
Capital expenditures											1,558	[4],[5]	6,343	[4],[5]	3,626	[4],[5]
Total assets	$ 117,536	[2],[4],[5]				$ 112,129	[2],[4],[5]				$ 117,536	[2],[4],[5]	$ 112,129	[2],[4],[5]	$ 134,086	[2],[4],[5]

[1] For those items included in the determination of segment EBITDA, the accounting policies of the segments are the same as those described in Note 1.

[2] Goodwill is included within total assets for each of the Company’s four operating segments.

[3] 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.

[4] 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.

[5] Corporate assets include cash, net deferred tax assets, debt issuance costs, equity investments, and corporate facility leasehold improvements and equipment.