Segment Reporting (Summary of Financial Information Concerning Company's Reportable Segments) (Detail) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Revenue		$ 531,937	$ 547,938	$ 531,312	$ 506,100	$ 526,213	$ 546,551	$ 524,693	$ 481,710	$ 2,117,287	$ 2,079,166	$ 1,928,795
Segment EBITDA	[1]									702,394	711,304	651,339
Depreciation and amortization										269,434	257,944	243,864
Capital expenditures										238,833	241,277	209,874
Total assets	[2]	5,121,798				5,245,267				5,121,798	5,245,267	5,057,617
Intercompany Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	[3]									(273,903)	(259,352)	(249,830)
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue										2,391,190	2,338,518	2,178,625
Segment EBITDA										700,461	718,738	651,567
Western [Member]
Segment Reporting Information [Line Items]
Revenue										880,393	823,922	805,790
Segment EBITDA	[1]									290,937	258,126	249,548
Depreciation and amortization										83,073	79,907	81,164
Capital expenditures										82,118	65,227	70,960
Total assets	[2]	1,498,296				1,482,474				1,498,296	1,482,474	1,487,409
Western [Member] | Intercompany Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	[3]									(103,890)	(96,194)	(99,974)
Western [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue										984,283	920,116	905,764
Segment EBITDA										290,937	258,126	249,548
Central [Member]
Segment Reporting Information [Line Items]
Revenue										589,667	561,480	510,928
Segment EBITDA	[1]									207,205	197,121	182,790
Depreciation and amortization										76,719	69,037	64,165
Capital expenditures										70,872	77,500	57,952
Total assets	[2]	1,216,985				1,187,505				1,216,985	1,187,505	1,173,089
Central [Member] | Intercompany Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	[3]									(71,235)	(68,094)	(62,438)
Central [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue										660,902	629,574	573,366
Segment EBITDA										207,205	197,121	182,790
Eastern [Member]
Segment Reporting Information [Line Items]
Revenue										433,457	393,821	371,772
Segment EBITDA	[1]									132,774	116,230	108,173
Depreciation and amortization										59,654	53,717	51,546
Capital expenditures										52,187	60,384	39,703
Total assets	[2]	1,176,671				852,963				1,176,671	852,963	807,124
Eastern [Member] | Intercompany Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	[3]									(87,234)	(80,162)	(76,072)
Eastern [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue										520,691	473,983	447,844
Segment EBITDA										132,774	116,230	108,173
Exploration and Production [Member]
Segment Reporting Information [Line Items]
Revenue										213,770	299,943	240,305	[4],[5]
Segment EBITDA	[1]									69,545	147,261	111,056	[4],[5]
Depreciation and amortization										47,129	52,709	44,099	[4],[5]
Capital expenditures										30,814	36,608	34,916	[4],[5]
Total assets	[2]	1,113,890				1,609,553				1,113,890	1,609,553	1,484,501	[4],[5]
Exploration and Production [Member] | Intercompany Revenues [Member]
Segment Reporting Information [Line Items]
Revenue	[3]									(11,544)	(14,902)	(11,346)	[4],[5]
Exploration and Production [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenue										225,314	314,845	251,651	[4],[5]
Segment EBITDA										69,545	147,261	111,056
Corporate [Member]
Segment Reporting Information [Line Items]
Segment EBITDA	[1],[4],[5]									1,933	(7,434)	(228)
Depreciation and amortization	[4],[5]									2,859	2,574	2,890
Capital expenditures	[4],[5]									2,842	1,558	6,343
Total assets	[2],[4],[5]	$ 115,956				$ 112,772				$ 115,956	$ 112,772	$ 105,494

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

[5] 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.