Annual report pursuant to Section 13 and 15(d)

Asset Retirement Obligations (Details)

v3.6.0.2
Asset Retirement Obligations (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Asset Retirement Obligation Rollforward    
Beginning of year $ 26,061 $ 23,445
Change in estimated liability (534) 2,101
Accretion expense 534 515
End of year $ 26,061 $ 26,061