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Income Taxes - Income Tax Expense (Benefit) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Current
Federal Income Taxes	$ 4,881	$ 5,156	$ 4,960
State Income Taxes	2,415	1,333	1,395
Deferred
Federal Income Taxes	11,450	8,859	8,065
State Income Taxes	3,751	3,167	4,410
North Dakota Wind Tax Credit Amortization, Net of Federal Taxes
Production Tax Credits	(1,250)	0	(3,111)
North Dakota Wind Tax Credit Amortization, Net of Federal Taxes	(1,033)	(1,033)	(1,033)
Investment Tax Credit Amortization	(8)	(41)	(98)
Income Tax Expense (Benefit)	$ 20,206	$ 17,441	$ 14,588

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