Quarterly report pursuant to Section 13 or 15(d)

Asset Retirement Obligations - Rollfoward (Details)

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Asset Retirement Obligations - Rollfoward (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Dec. 31, 2019
Asset Retirement Obligation Rollforward          
Beginning of period     $ 30,972 $ 30,384 $ 30,384
Change in estimated liability         11
Accretion expense $ 143 $ 144 288 $ 287 577
End of period $ 31,260   $ 31,260   $ 30,972