Changes in Asset Retirement Obligations (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Asset Retirement Obligation Disclosure [Abstract]
Asset retirement obligations as of the beginning of the period, including current portion			$ 488,300,000
Liabilities incurred			16,500,000
Liabilities settled			(61,200,000)
Accretion expense	8,431,000	8,405,000	25,012,000	24,926,000
Asset retirement obligations as of the end of the period, including current portion	$ 468,600,000		$ 468,600,000