Reconciliations of Asset Retirement Obligations (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Asset Retirement Obligations
Balance at beginning of year	$ 153,979,000	$ 162,730,000
Liabilities incurred	127,000	1,738,000
Liabilities settled	(2,993,000)	(16,630,000)
Accretion expense	7,956,000	8,195,000	8,641,000
Revisions up (down), net	(8,997,000)	(2,054,000)
Balance at end of year	$ 150,072,000	$ 153,979,000	$ 162,730,000