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REVENUE - Remaining performance obligations (Details) - Operating Segment $ in Thousands	Sep. 30, 2025 USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue expected to be recognized	$ 921,700
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2025-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue expected to be recognized	$ 83,830
Unsatisfied or partially unsatisfied performance obligation, period	3 months
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 expected to be recognized	$ 385,460
Unsatisfied or partially unsatisfied performance obligation, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2027-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue expected to be recognized	$ 310,550
Unsatisfied or partially unsatisfied performance obligation, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2028-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue expected to be recognized	$ 124,530
Unsatisfied or partially unsatisfied performance obligation, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2029-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue expected to be recognized	$ 17,330
Unsatisfied or partially unsatisfied performance obligation, period	1 year
Electric Operations | Delivered energy
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue expected to be recognized	$ 405,000
Electric Operations | Delivered energy | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2025-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue expected to be recognized	$ 43,780
Unsatisfied or partially unsatisfied performance obligation, period	3 months
Electric Operations | Delivered energy | 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 expected to be recognized	$ 172,360
Unsatisfied or partially unsatisfied performance obligation, period	1 year
Electric Operations | Delivered energy | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2027-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue expected to be recognized	$ 117,300
Unsatisfied or partially unsatisfied performance obligation, period	1 year
Electric Operations | Delivered energy | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2028-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue expected to be recognized	$ 57,700
Unsatisfied or partially unsatisfied performance obligation, period	1 year
Electric Operations | Delivered energy | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2029-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue expected to be recognized	$ 13,860
Unsatisfied or partially unsatisfied performance obligation, period	1 year
Electric Operations | Energy Capacity
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue expected to be recognized	$ 166,720
Electric Operations | Energy Capacity | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2025-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue expected to be recognized	$ 12,980
Unsatisfied or partially unsatisfied performance obligation, period	3 months
Electric Operations | Energy Capacity | 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 expected to be recognized	$ 61,540
Unsatisfied or partially unsatisfied performance obligation, period	1 year
Electric Operations | Energy Capacity | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2027-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue expected to be recognized	$ 51,400
Unsatisfied or partially unsatisfied performance obligation, period	1 year
Electric Operations | Energy Capacity | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2028-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue expected to be recognized	$ 37,330
Unsatisfied or partially unsatisfied performance obligation, period	1 year
Electric Operations | Energy Capacity | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2029-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue expected to be recognized	$ 3,470
Unsatisfied or partially unsatisfied performance obligation, period	1 year
Coal Operations
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue expected to be recognized	$ 349,980
Coal Operations | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2025-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue expected to be recognized	$ 27,070
Unsatisfied or partially unsatisfied performance obligation, period	3 months
Coal Operations | 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 expected to be recognized	$ 151,560
Unsatisfied or partially unsatisfied performance obligation, period	1 year
Coal Operations | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2027-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue expected to be recognized	$ 141,850
Unsatisfied or partially unsatisfied performance obligation, period	1 year
Coal Operations | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2028-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue expected to be recognized	$ 29,500
Unsatisfied or partially unsatisfied performance obligation, period	1 year
Coal Operations | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2029-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Unsatisfied or partially unsatisfied performance obligation, period	1 year

X

- Definition

Amount of transaction price allocated to performance obligation that has not been recognized as revenue.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (a)
-SubTopic 10
-Topic 606
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-13

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- Definition

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+ References

No definition available.

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- Definition

Period in which remaining performance obligation is expected to be recognized as revenue, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (b)(1)
-SubTopic 10
-Topic 606
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-13

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