Quarterly report pursuant to Section 13 or 15(d)

CRUDE OIL AND NATURAL GAS PROPERTIES - Narrative (Details)

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CRUDE OIL AND NATURAL GAS PROPERTIES - Narrative (Details)
$ in Millions
6 Months Ended 12 Months Ended
Jun. 30, 2018
USD ($)
a
Jun. 30, 2017
USD ($)
Dec. 31, 2017
USD ($)
a
Acquisitions [Abstract]      
Capital expenditures and property purchases $ 80.1   $ 85.0
Unproved Properties [Abstract]      
Unproven leasehold interests (in acres) | a 11,371   14,377
Anticipated future period over which excluded costs will become subject to depletion 5 years    
Impairment costs related to expired leases subject to depletion $ 0.1 $ 0.1