Note 9 - Equity - Common Stock (Details) $ / shares in Units, $ in Millions			3 Months Ended		12 Months Ended
	Jun. 21, 2023 shares	Jun. 12, 2023 Vote $ / shares	Jun. 30, 2023 USD ($)	Jun. 30, 2022 $ / T	Dec. 31, 2023 D $ / shares	Dec. 31, 2022 $ / shares
Class of Stock [Line Items]
Common stock, par value (in dollars per share)					$ 0.01	$ 0.01
Percent of royalty fee income				3.00%
Price per ton of coal processed at preparation plants | $ / T				5.00
Price per ton of loaded coal at rail load-out facilities | $ / T				2.50
Percentage dividend from financial performance of carbon				20
Threshold trading days volume weighted average price | D					20
Dividend per share of tracking stock		$ 11.00
Retained Earnings
Class of Stock [Line Items]
Adjustments relating to restructuring of equity | $			$ (102.9)
Additional Paid-in Capital.
Class of Stock [Line Items]
Adjustments relating to restructuring of equity | $			102.9
Class A Common Stock
Class of Stock [Line Items]
Common stock, par value (in dollars per share)					$ 0.01
Number of votes per common share | Vote		1
Class B Common Stock
Class of Stock [Line Items]
Common stock, par value (in dollars per share)					$ 0.01
Distribution ratio	0.2
Percentage dividend from financial performance of carbon					20
Number of votes per common share | Vote		1
Number of shares issued | shares	8,201,956
Class B Common Stock | Common Stock
Class of Stock [Line Items]
Adjustments relating to restructuring of equity | $			$ 102.9