Annual report pursuant to Section 13 and 15(d)

Commitments - Additional Information (Details)

v3.10.0.1
Commitments - Additional Information (Details) - USD ($)
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Commitments [Line Items]      
Minimum future lease payments due under noncancelable operating leases, 2019 $ 1,500,000    
Minimum future lease payments due under noncancelable operating leases, 2020 1,600,000    
Minimum future lease payments due under noncancelable operating leases, 2021 1,600,000    
Minimum future lease payments due under noncancelable operating leases, 2022 1,600,000    
Minimum future lease payments due under noncancelable operating leases, thereafter 0    
Total rent expense 3,400,000 $ 3,000,000 $ 3,200,000
Escrow related to Purchase and Sale Agreement 49,500,000 49,500,000  
Collateral deposits 6,900,000    
Expenses related to surety bonds 5,900,000 $ 5,700,000 $ 4,300,000
Drilling Rig Commitments      
Commitments [Line Items]      
Minimum future lease payments due under noncancelable operating leases, 2018 9,700,000    
Heidelberg Field Acquisition      
Commitments [Line Items]      
Future estimated payment, 2019 4,900,000    
Future estimated payment, 2020 4,000,000    
Future estimated payment, 2021 2,300,000    
Future estimated payment, 2022 1,700,000    
Future estimated payment, 2023 1,200,000    
Future estimated payment, thereafter 2,000,000    
Surety Bonds      
Commitments [Line Items]      
Future estimated payment, 2019 4,500,000    
Future estimated payment, 2020 4,200,000    
Future estimated payment, 2021 3,900,000    
Future estimated payment, 2022 3,900,000    
Future estimated payment, 2023 3,800,000    
Future estimated payment, thereafter 33,600,000    
Surety Bonds | Other commitment      
Commitments [Line Items]      
Security requirement minimum 64,000,000    
Security requirement maximum 94,000,000    
Surety Bonds | Total E&P Member      
Commitments [Line Items]      
Security amount requirement 88,500,000    
Escrow related to Purchase and Sale Agreement 0    
Additional security requirements for 2019 91,000,000    
Additional security requirements for 2023 103,000,000    
Annual increment in threshold $ 3,000,000