Quarterly report pursuant to Section 13 or 15(d)

Asset Retirement Obligations (Details)

v3.7.0.1
Asset Retirement Obligations (Details) - USD ($)
$ in Thousands
6 Months Ended 12 Months Ended
Jun. 30, 2017
Dec. 31, 2016
Asset Retirement Obligation Rollforward    
Estimated Future Cash Undiscounted Amount $ 27,300  
Beginning of year 26,061 $ 26,061
Change in estimated liability 815 (534)
Accretion expense 266 534
End of year $ 27,142 $ 26,061