Note 6 - Revenue 2 (Details Textual) $ in Millions	Sep. 30, 2023 USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-10-01 | Fixed-Price Contract [Member]
Revenue, Remaining Performance Obligation, Amount	$ 426.1
Revenue, Remaining Performance Obligation, Percentage	0.25%
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-10-01 | Energy Obligations [Member]
Revenue, Remaining Performance Obligation, Amount	$ 312.0
Revenue, Remaining Performance Obligation, Percentage	31.00%
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period (Year)	2028 years
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-10-01 | Contract Capacity [Member]
Revenue, Remaining Performance Obligation, Amount	$ 204.0
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period (Year)	2028 years
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2024-01-01 | Fixed-Price Contract [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period (Year)	3 years
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2025-01-01 | Coal Sales [Member]
Revenue, Remaining Performance Obligation, Amount	$ 155.0
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period (Year)	1 year