Annual report pursuant to Section 13 and 15(d)

Asset Retirement Obligations - Rollfoward (Details)

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Asset Retirement Obligations - Rollfoward (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Asset Retirement Obligation Rollforward      
Beginning of period $ 30,384 $ 27,036  
Change in estimated liability 11 2,840  
Accretion expense 577 508 $ 527
End of period $ 30,972 $ 30,384 $ 27,036