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Significant Accounting Policies - Impairment of Proved and Unproved Properties (Details)
a in Thousands, $ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
a
well
Dec. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
a
well
Dec. 31, 2018
USD ($)
Productive Wells [Line Items]          
Impairment of proved and unproved properties $ 61,849 $ 327,400 $ 61,849 $ 327,400 $ 0
Impairment of unproved properties $ 473,045 $ 119,429 0 119,429 0
Exploration and Production Related Other Costs          
Productive Wells [Line Items]          
Exploration expense     $ 14,994 $ 44,380 $ 12,033
Marcellus Shale | Central Pennsylvania          
Productive Wells [Line Items]          
Number of operated wells | well   56   56  
Net acres of land | a   51   51