Segment Reporting (Changes in Goodwill by Reportable Segment) (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Goodwill [Line Items]
Goodwill, Beginning Balance	$ 1,693,789	$ 1,675,154
Goodwill acquired	28,535	1,559
Goodwill divested		(143)
Goodwill adjustments		(844)
Goodwill, Ending Balance	1,722,324	1,675,726
Western [Member]
Goodwill [Line Items]
Goodwill, Beginning Balance	372,915	372,915
Goodwill acquired	75
Goodwill, Ending Balance	372,990	372,915
Central [Member]
Goodwill [Line Items]
Goodwill, Beginning Balance	460,381	459,054
Goodwill acquired	615	1,559
Goodwill divested		(143)
Goodwill adjustments		(843)
Goodwill, Ending Balance	460,996	459,627
Eastern [Member]
Goodwill [Line Items]
Goodwill, Beginning Balance	392,423	380,570
Goodwill acquired	6,517
Goodwill adjustments		(1)
Goodwill, Ending Balance	398,940	380,569
Exploration and Production [Member]
Goodwill [Line Items]
Goodwill, Beginning Balance	468,070	462,615
Goodwill acquired	21,328
Goodwill, Ending Balance	$ 489,398	$ 462,615