XML 57 R62.htm IDEA: XBRL DOCUMENT v2.4.0.6
Asset Retirement Obligation - Information Related to Asset Retirement Obligation (Detail) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Asset Retirement Obligations [Line Items]    
Beginning Balance $ 180,662 $ 173,673
Additions to asset retirement obligations 5,434 708
Accretion expense 10,871 10,573
Liabilities settled (7,308) (4,292)
Ending Balance $ 189,659 $ 180,662