Annual report pursuant to Section 13 and 15(d)

Accrued Liabilities and Other Non-Current Liabilities, Changes in Asset Retirement Obligation (Details)

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Accrued Liabilities and Other Non-Current Liabilities, Changes in Asset Retirement Obligation (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Asset Retirement Obligation    
Balance at beginning of year $ 10,906 $ 12,328
Accretion expense 804 775
Revisions in estimated cash flows 934 (2,128)
Retirements (181) (213)
Additions 0 144
Balance at end of year $ 12,463 $ 10,906