Changes in Asset Retirement Obligations (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Asset Retirement Obligation Disclosure [Abstract]
Asset retirement obligations as of the beginning of the year, including current portion	$ 488,302	$ 425,779	$ 373,920
Liabilities incurred	19,043	3,869	7,993
Liabilities settled	(79,695)	(67,641)	(63,226)
Liabilities assumed		15,263	59,441
Divestment of properties	(9,245)	(7,563)	(10,900)
Accretion expense	33,575	33,331	30,764
Revision of estimates	50,533	85,264	27,787
Asset retirement obligations as of the end of the year, including current portion	$ 502,513	$ 488,302	$ 425,779