Quarterly report pursuant to Section 13 or 15(d)

Asset Retirement Obligation (Details)

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Asset Retirement Obligation (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
Mar. 31, 2017
Asset retirement obligation $ 767   $ 767   $ 745 $ 700
Accretion expense 11,000 $ 11,000 23,000 $ 21,000    
Other Assets            
Contributed to the Trust Fund $ 670,000   $ 670,000