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Note 14 - Income Taxes (Details Textual) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%	21.00%	35.00%
Increase (Decrease) in Percentage of Production Tax Credits		(53.00%)
Wind Tax Credits, Amortization Period		25 years
Period for Unrecognized Tax Benefits Not Expected Change	12 months
Unrecognized Tax Benefits, Interest on Income Taxes Accrued	$ 0
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability	$ 0	$ 0	$ 1,756
Domestic Tax Authority [Member]
Open Tax Year	2016 2017 2018 2019
State and Local Jurisdiction [Member] | Minnesota Department of Revenue [Member]
Open Tax Year	2015 2016 2017 2018 2019
State and Local Jurisdiction [Member] | North Dakota Department of Revenue [Member]
Open Tax Year	2016 2017 2018 2019

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- Definition

Represents the increase (decrease) in the percentage of production tax credits during the period.

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Represents the period over which there is no significant expected change in the unrecognized tax benefits.

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Represents the amortization period for wind tax credits.

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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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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.

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Tax year that remains open to examination under enacted tax laws, in CCYY format.

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Amount of interest expense accrued for an underpayment of income taxes.

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