Asset Retirement Obligations - Changes in Asset Retirement Obligations (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Asset Retirement Obligation Disclosure [Abstract]
Asset retirement obligations as of the beginning of the period, including current portion			$ 502,500,000
Liabilities incurred			25,700,000
Liabilities settled			(24,900,000)
Divestment of properties			(10,400,000)
Accretion expense	7,733,000	8,318,000	15,288,000	16,581,000
Asset retirement obligations as of the end of the period, including current portion	$ 508,200,000		$ 508,200,000