REVENUE FROM CONTRACTS WITH CUSTOMERS - Coal supply contracts (Details) $ in Thousands	Mar. 31, 2020 USD ($)
Performance obligations unsatisfied or partially unsatisfied
Total	$ 2,177,006
Illinois Basin
Performance obligations unsatisfied or partially unsatisfied
Total	1,679,338
Appalachia
Performance obligations unsatisfied or partially unsatisfied
Total	497,668
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01
Performance obligations unsatisfied or partially unsatisfied
Total	$ 969,965
Expected timing of satisfaction period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01 | Illinois Basin
Performance obligations unsatisfied or partially unsatisfied
Total	$ 621,541
Expected timing of satisfaction period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01 | Appalachia
Performance obligations unsatisfied or partially unsatisfied
Total	$ 348,424
Expected timing of satisfaction period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01
Performance obligations unsatisfied or partially unsatisfied
Total	$ 680,193
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	$ 588,838
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	$ 91,355
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	$ 252,653
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	$ 194,764
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	$ 57,889
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