Note 7 - Income Taxes (Details Textual) - State and Local Jurisdiction [Member] - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2024	Sep. 30, 2023	Jun. 30, 2023
Iowa, Kansas, and Arkansas Department of Revenue [Member]
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability		$ (41)
Arkansas Department of Revenue [Member]
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability	$ (8)
Nebraska Department of Revenue [Member]
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability			$ (73)