REVENUE FROM CONTRACTS WITH CUSTOMERS - Supply contracts (Details) $ in Thousands	Dec. 31, 2024 USD ($)
Performance obligations unsatisfied or partially unsatisfied
Total	$ 3,328,509
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2025-01-01
Performance obligations unsatisfied or partially unsatisfied
Total	$ 1,542,169
Expected timing of satisfaction period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2026-01-01
Performance obligations unsatisfied or partially unsatisfied
Total	$ 734,052
Expected timing of satisfaction period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2027-01-01
Performance obligations unsatisfied or partially unsatisfied
Total	$ 418,461
Expected timing of satisfaction period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2028-01-01
Performance obligations unsatisfied or partially unsatisfied
Total	$ 633,827
Expected timing of satisfaction period	1 year
Illinois Basin Coal
Performance obligations unsatisfied or partially unsatisfied
Total	$ 2,649,488
Illinois Basin Coal | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2025-01-01
Performance obligations unsatisfied or partially unsatisfied
Total	$ 993,263
Expected timing of satisfaction period	1 year
Illinois Basin Coal | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2026-01-01
Performance obligations unsatisfied or partially unsatisfied
Total	$ 634,042
Expected timing of satisfaction period	1 year
Illinois Basin Coal | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2027-01-01
Performance obligations unsatisfied or partially unsatisfied
Total	$ 388,356
Expected timing of satisfaction period	1 year
Illinois Basin Coal | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2028-01-01
Performance obligations unsatisfied or partially unsatisfied
Total	$ 633,827
Expected timing of satisfaction period	1 year
Appalachia Coal
Performance obligations unsatisfied or partially unsatisfied
Total	$ 679,021
Appalachia Coal | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2025-01-01
Performance obligations unsatisfied or partially unsatisfied
Total	$ 548,906
Expected timing of satisfaction period	1 year
Appalachia Coal | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2026-01-01
Performance obligations unsatisfied or partially unsatisfied
Total	$ 100,010
Expected timing of satisfaction period	1 year
Appalachia Coal | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2027-01-01
Performance obligations unsatisfied or partially unsatisfied
Total	$ 30,105
Expected timing of satisfaction period	1 year
Appalachia Coal | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2028-01-01
Performance obligations unsatisfied or partially unsatisfied
Expected timing of satisfaction period	1 year