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Note 6 - Income Taxes (Details Textual) - State and Local Jurisdiction [Member] - USD ($) $ in Millions | 3 Months Ended
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Sep. 30, 2023 | Jun. 30, 2023 | Sep. 30, 2022 | Jun. 30, 2022
Iowa, Kansas, and Arkansas [Member] |  |  |  |
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability | $ (41) |  |  |
Iowa, Arkansas, and Idaho Department of Revenue [Member] |  |  |  |
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability |  |  | $ (40) |
Nebraska Department of Revenue [Member] |  |  |  |
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability |  | $ (73) |  | $ (55)

X
- Definition Amount of income tax expense (benefit) from continuing operations attributable to an adjustment of a deferred tax liability or asset for enacted changes in tax laws or rates or a change in the tax status of the entity. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 740 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 9 -Subparagraph (g) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482685/740-10-50-9 + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
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X
- Details
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X
- Details
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X
- Details
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