Note 14 - Income Tax Expense - Continuing Operations - Effective Income Tax Rate (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Income Before Income Taxes – Continuing Operations	$ 22,614	$ 20,639	$ 48,506	$ 40,621
Tax Computed at Company’s Net Composite Federal and State Statutory Rate (39%)	8,819	8,049	18,917	15,842
Increases (Decreases) in Tax from:
Federal Production Tax Credits	(2,010)	(1,885)	(4,062)	(3,571)
Excess Tax Deduction – 2014 Performance Share Awards			(697)
Section 199 Domestic Production Activities Deduction	(330)	(94)	(660)	(198)
Corporate-Owned Life Insurance	(207)	(480)	(501)	(572)
North Dakota Wind Tax Credit Amortization – Net of Federal Taxes	(213)	(213)	(425)	(425)
Employee Stock Ownership Plan Dividend Deduction	(173)	(157)	(345)	(315)
Other Items – Net	11	(137)	33	(186)
Income Tax Expense—Continuing Operations	$ 5,897	$ 5,083	$ 12,260	$ 10,575
Effective Income Tax Rate – Continuing Operations	26.10%	24.60%	25.30%	26.00%

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Represents the income Tax Expense (Benefit), at federal and state statutory income tax rate.

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Income Tax Reconciliation Increases (Decreases) [Abstract]

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Represents information about income tax reconciliation production tax credit.

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Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Amount of income (loss) from continuing operations, including income (loss) from equity method investments, before deduction of income tax expense (benefit), and income (loss) attributable to noncontrolling interest.

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Amount of the difference between reported income tax expense (benefit) and the expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to deduction for qualified production activity.

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to tax exempt income, equity in earnings (loss) of an unconsolidated subsidiary, minority noncontrolling interest income (loss), tax holiday, disposition of a business, disposition of an asset, repatriation of foreign earnings, repatriation of foreign earnings jobs creation act of 2004, increase (decrease) in enacted tax rate, prior year income taxes, increase (decrease) in deferred tax asset valuation allowance, and other adjustments.

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