Segment Reporting (Changes in Goodwill by Reportable Segment) (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2015	Dec. 31, 2014
Goodwill [Line Items]
Goodwill, Beginning Balance		$ 1,693,789	$ 1,675,154
Goodwill acquired		140,822	18,778
Goodwill divested			(143)
Impairment loss		(411,786)
Goodwill, Ending Balance		1,422,825	1,693,789
Western [Member]
Goodwill [Line Items]
Goodwill, Beginning Balance		372,915	372,915
Goodwill acquired		905
Impairment loss		0	0
Goodwill, Ending Balance		373,820	372,915
Central [Member]
Goodwill [Line Items]
Goodwill, Beginning Balance		460,381	459,054
Goodwill acquired		12,044	1,470
Goodwill divested			(143)
Impairment loss		0	0
Goodwill, Ending Balance		472,425	460,381
Eastern [Member]
Goodwill [Line Items]
Goodwill, Beginning Balance		392,423	380,570
Goodwill acquired		106,814	11,853
Impairment loss		0	0
Goodwill, Ending Balance		499,237	392,423
Exploration and Production [Member]
Goodwill [Line Items]
Goodwill, Beginning Balance		468,070	462,615
Goodwill acquired		21,059	5,455
Impairment loss	$ (411,786)	(411,786)
Goodwill, Ending Balance		$ 77,343	$ 468,070