REVENUE FROM CONTRACTS WITH CUSTOMERS - Supply contracts (Details) $ in Thousands	Dec. 31, 2022 USD ($)
Performance obligations unsatisfied or partially unsatisfied
Total	$ 4,113,892
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01
Performance obligations unsatisfied or partially unsatisfied
Total	$ 1,951,694
Expected timing of satisfaction period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2024-01-01
Performance obligations unsatisfied or partially unsatisfied
Total	$ 1,263,472
Expected timing of satisfaction period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2025-01-01
Performance obligations unsatisfied or partially unsatisfied
Total	$ 653,526
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	$ 245,200
Expected timing of satisfaction period	1 year
Illinois Basin
Performance obligations unsatisfied or partially unsatisfied
Total	$ 2,549,577
Illinois Basin | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01
Performance obligations unsatisfied or partially unsatisfied
Total	$ 1,230,551
Expected timing of satisfaction period	1 year
Illinois Basin | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2024-01-01
Performance obligations unsatisfied or partially unsatisfied
Total	$ 730,825
Expected timing of satisfaction period	1 year
Illinois Basin | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2025-01-01
Performance obligations unsatisfied or partially unsatisfied
Total	$ 344,601
Expected timing of satisfaction period	1 year
Illinois Basin | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2026-01-01
Performance obligations unsatisfied or partially unsatisfied
Total	$ 243,600
Expected timing of satisfaction period	1 year
Appalachia
Performance obligations unsatisfied or partially unsatisfied
Total	$ 1,564,315
Appalachia | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01
Performance obligations unsatisfied or partially unsatisfied
Total	$ 721,143
Expected timing of satisfaction period	1 year
Appalachia | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2024-01-01
Performance obligations unsatisfied or partially unsatisfied
Total	$ 532,647
Expected timing of satisfaction period	1 year
Appalachia | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2025-01-01
Performance obligations unsatisfied or partially unsatisfied
Total	$ 308,925
Expected timing of satisfaction period	1 year
Appalachia | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2026-01-01
Performance obligations unsatisfied or partially unsatisfied
Total	$ 1,600
Expected timing of satisfaction period	1 year