Annual report pursuant to Section 13 and 15(d)

Summary of Significant Accounting Principles (Asset Retirement Obligations) (Details)

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Summary of Significant Accounting Principles (Asset Retirement Obligations) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Balance at beginning of period $ 40,461 $ 0
Liability acquired   38,686
Liability incurred 217 113
Accretion expense 1,596 1,662
Liability settled (74) 0
Balance at end of period $ 42,200 $ 40,461