REVENUE FROM CONTRACTS WITH CUSTOMERS - Coal supply contracts (Details) $ in Thousands	Jun. 30, 2020 USD ($)
Performance obligations unsatisfied or partially unsatisfied
Total	$ 1,949,034
Illinois Basin
Performance obligations unsatisfied or partially unsatisfied
Total	1,558,199
Appalachia
Performance obligations unsatisfied or partially unsatisfied
Total	390,835
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-07-01
Performance obligations unsatisfied or partially unsatisfied
Total	$ 702,329
Expected timing of satisfaction period	6 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-07-01 | Illinois Basin
Performance obligations unsatisfied or partially unsatisfied
Total	$ 462,549
Expected timing of satisfaction period	6 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-07-01 | Appalachia
Performance obligations unsatisfied or partially unsatisfied
Total	$ 239,780
Expected timing of satisfaction period	6 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01
Performance obligations unsatisfied or partially unsatisfied
Total	$ 692,118
Expected timing of satisfaction period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01 | Illinois Basin
Performance obligations unsatisfied or partially unsatisfied
Total	$ 598,941
Expected timing of satisfaction period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01 | Appalachia
Performance obligations unsatisfied or partially unsatisfied
Total	$ 93,177
Expected timing of satisfaction period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01
Performance obligations unsatisfied or partially unsatisfied
Total	$ 280,392
Expected timing of satisfaction period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01 | Illinois Basin
Performance obligations unsatisfied or partially unsatisfied
Total	$ 222,514
Expected timing of satisfaction period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01 | Appalachia
Performance obligations unsatisfied or partially unsatisfied
Total	$ 57,878
Expected timing of satisfaction period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01
Performance obligations unsatisfied or partially unsatisfied
Total	$ 274,195
Expected timing of satisfaction period
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01 | Illinois Basin
Performance obligations unsatisfied or partially unsatisfied
Total	$ 274,195
Expected timing of satisfaction period
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01 | Appalachia
Performance obligations unsatisfied or partially unsatisfied
Expected timing of satisfaction period