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 12 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Dec. 31, 2019
Asset Retirement Obligation Rollforward      
Beginning of period $ 30,972 $ 30,384 $ 30,384
Change in estimated liability     11
Accretion expense 145 $ 143 577
End of period $ 31,117   $ 30,972