Annual report pursuant to Section 13 and 15(d)

Asset Retirement Obligations - Estimated Fair Value of Company's Asset Retirement Obligations (Detail)

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Asset Retirement Obligations - Estimated Fair Value of Company's Asset Retirement Obligations (Detail) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Estimated fair value of the Company's asset retirement obligations      
Balances at January 1, $ 10,368 $ 14,528 $ 13,425
Accretion Expense 643 814 1,014
Additions 453 770 96
Revisions 0 (5,744) (7)
Balance December 31, $ 11,464 $ 10,368 $ 14,528