Segment Reporting (Changes in Goodwill by Reportable Segment) (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013
Goodwill [Line Items]
Goodwill, Beginning Balance	$ 1,675,154	$ 1,636,557
Goodwill transferred
Goodwill acquired	1,559	1,603
Goodwill divested	(143)
Goodwill adjustments	(844)
Goodwill, Ending Balance	1,675,726	1,638,160
Western [Member]
Goodwill [Line Items]
Goodwill, Beginning Balance	372,915	373,143
Goodwill transferred
Goodwill acquired		521
Goodwill adjustments
Goodwill, Ending Balance	372,915	373,664
Central [Member]
Goodwill [Line Items]
Goodwill, Beginning Balance	459,054	430,412
Goodwill transferred		(9,196)
Goodwill acquired	1,559	52
Goodwill divested	(143)
Goodwill adjustments	(843)
Goodwill, Ending Balance	459,627	421,268
Eastern [Member]
Goodwill [Line Items]
Goodwill, Beginning Balance	380,570	380,561
Goodwill transferred
Goodwill acquired		149
Goodwill adjustments	(1)
Goodwill, Ending Balance	380,569	380,710
Exploration and Production [Member]
Goodwill [Line Items]
Goodwill, Beginning Balance	462,615	452,441
Goodwill transferred		9,196
Goodwill acquired		881
Goodwill adjustments
Goodwill, Ending Balance	$ 462,615	$ 462,518