Note 6 - Revenue 2 (Details Textual) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2024-01-01 | Fixed-Price Contract [Member]
Revenue, Remaining Performance Obligation, Percentage	55.00%
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period (Year)	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2024-01-01 | Contracts with Price Reopeners [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period (Year)		1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2024-01-01 | Coal Sales [Member]
Revenue, Remaining Performance Obligation, Amount	$ 155.0
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2024-01-01 | Energy Obligations [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period (Year)	1 year
Revenue, Remaining Performance Obligation, Amount	$ 186.6
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2024-01-01 | Energy Obligations [Member] | Hoosier [Member]
Revenue, Remaining Performance Obligation, Amount	$ 115.6
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2024-01-01 | Contract Capacity [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period (Year)	1 year
Revenue, Remaining Performance Obligation, Amount	$ 144.6
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2024-01-01 | Contract Capacity [Member] | Hoosier [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period (Year)	1 year
Revenue, Remaining Performance Obligation, Amount	$ 41.6
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2028-01-01 | Contract Capacity [Member] | Hoosier [Member]
Revenue, Remaining Performance Obligation, Amount	$ 60.9