REVENUE - Remaining performance obligations (Details) $ in Thousands	Sep. 30, 2024 USD ($)
Energy Obligations [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2024-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 320,280
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period (Year)	3 months
Revenue, Remaining Performance Obligation, Percentage	9.90%
Energy Obligations [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2025-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period (Year)	4 years
Revenue, Remaining Performance Obligation, Percentage	90.10%
Energy Obligations [Member] | Hoosier [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2024-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 70,100
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period (Year)	4 years 3 months
Contract Capacity [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2024-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 140,200
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period (Year)	5 years 3 months
Contract Capacity [Member] | Hoosier [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 186,600
Contract Capacity [Member] | Hoosier [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2024-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 25,000
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period (Year)	1 year 3 months
Contract Capacity [Member] | Hoosier [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2026-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 60,900
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period (Year)	3 years
Additional Obligations [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2024-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 134,100
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period (Year)	5 years 3 months