Note 14 - Income Taxes - Effective Income Tax Rate (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Tax Computed at Federal Statutory Rate	$ 21,901	$ 20,356	$ 34,893
Increases (Decreases) in Tax from:
State Income Taxes Net of Federal Income Tax Expense	3,561	5,210	4,368
Differences Reversing in Excess of Federal Rates	(3,357)	(3,432)	551
Permanent Differences, R&D Tax Credits, Unitary Tax and Other Adjustments	(1,315)	(1,864)	(1,873)
North Dakota Wind Tax Credit Amortization – Net of Federal Taxes	(1,033)	(1,033)	(850)
Corporate-owned Life Insurance	(749)	(3)	(845)
Excess Tax deduction – Equity Method Stock Awards	(744)	(708)	(751)
Allowance for Funds Used During Construction – Equity	(501)	(431)	(322)
Employee Stock Ownership Plan Dividend Deduction	(281)	(298)	(509)
Investment Tax Credit Amortization	(41)	(98)	(164)
Federal Production Tax Credits (PTCs)	0	(3,111)	(7,527)
Section 199 Domestic Production Activities Deduction	0	0	(1,471)
Effect of TCJA Tax Rate Reduction on Value of Net Deferred Tax Assets	0	0	1,756
Income Tax Expense	$ 17,441	$ 14,588	$ 27,256
Overall Effective Federal and State Income Tax Rate	16.70%	15.00%	27.30%
Current Federal Income Taxes	$ 5,156	$ 4,960	$ 4,434
Current State Income Taxes	1,333	1,395	1,128
Deferred Federal Income Taxes	8,859	8,065	25,648
Deferred State Income Taxes	3,167	4,410	4,587
Federal PTCs	0	(3,111)	(7,527)
North Dakota Wind Tax Credit Amortization – Net of Federal Taxes	(1,033)	(1,033)	(850)
Investment Tax Credit Amortization	(41)	(98)	(164)
Total	17,441	14,588	27,256
Total Income Before Income Taxes	$ 104,288	$ 96,933	$ 99,695

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 corporate-owned life insurance.

No definition available.