Changes in Goodwill by Reportable Segment (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Goodwill [Line Items]
Goodwill, Beginning Balance	$ 1,636,557	$ 1,116,888
Goodwill acquired	1,603	65,709
Goodwill divested		(496)
Goodwill, Ending Balance	1,638,160	1,182,101
Western
Goodwill [Line Items]
Goodwill, Beginning Balance	373,143	313,038
Goodwill acquired	521	60,538
Goodwill, Ending Balance	373,664	373,576
Central
Goodwill [Line Items]
Goodwill, Beginning Balance	430,412	424,223
Goodwill transferred	(9,196)
Goodwill acquired	52	5,153
Goodwill divested		(496)
Goodwill, Ending Balance	421,268	428,880
Eastern
Goodwill [Line Items]
Goodwill, Beginning Balance	380,561	379,627
Goodwill acquired	149	18
Goodwill, Ending Balance	380,710	379,645
Exploration and Production
Goodwill [Line Items]
Goodwill, Beginning Balance	452,441
Goodwill transferred	9,196
Goodwill acquired	881
Goodwill, Ending Balance	$ 462,518