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Asset Retirement Obligation (Schedule Of ARO Activity) (Detail) (Asset Retirement Obligation Costs [Member], USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Asset Retirement Obligation Costs [Member]
   
Beginning balance $ 56,849 $ 51,295
Adjustment due to change in estimate of related costs 748 3,570
Accretion of liability 5,106 4,920
Liabilities settled (3,323) (2,936)
Ending balance $ 59,380 $ 56,849