Changes in Asset Retirement Obligations (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 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			(37,300,000)
Accretion expense	8,318,000	8,255,000	16,581,000	16,521,000
Asset retirement obligations as of the end of the period, including current portion	$ 484,100,000		$ 484,100,000