VARIABLE INTEREST ENTITIES - Cavalier Minerals (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Oct. 06, 2015	Nov. 10, 2014
Cavalier Minerals | All Dale I
Variable Interest Entities
Noncontrolling ownership interest (as a percent)					71.70%
Cavalier Minerals | All Dale II
Variable Interest Entities
Noncontrolling ownership interest (as a percent)					72.80%
Funding Commitments | Cavalier Minerals
Variable Interest Entities
Funding commitment				$ 100,000
Funding Commitments | Cavalier Minerals | All Dale I
Variable Interest Entities
Funding commitment					$ 49,000
Funding Commitments | Alliance Minerals | Cavalier Minerals
Variable Interest Entities
Funding commitment	$ 888	$ 6,923	$ 80,502
Funding Commitments | Bluegrass Minerals | Cavalier Minerals
Variable Interest Entities
Funding commitment	$ 37	$ 288	$ 3,354
Initial Funding Commitment | Bluegrass Minerals | Cavalier Minerals
Variable Interest Entities
Funding commitment					2,000
Initial Funding Commitment | Variable Interest Entity, Primary Beneficiary | Alliance Minerals | Cavalier Minerals
Variable Interest Entities
Funding commitment					$ 48,000
Additional Funding Commitment | Bluegrass Minerals | Cavalier Minerals
Variable Interest Entities
Funding commitment				4,000
Additional Funding Commitment | Variable Interest Entity, Primary Beneficiary | Alliance Minerals | Cavalier Minerals
Variable Interest Entities
Funding commitment				$ 96,000