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Income Taxes - Effective Income Tax Rate (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Income Tax Disclosure [Abstract]
Income Taxes at Federal Statutory Rate	$ 24,508	$ 22,418	$ 61,849	$ 64,748
Income Taxes at Federal Statutory Rate	21.00%	21.00%	21.00%	21.00%
Increases (Decreases) in Tax from:
State Taxes on Income, Net of Federal Tax	$ 5,832	$ 5,338	$ 14,725	$ 15,416
Production Tax Credits (PTCs)	(5,192)	(3,828)	(14,421)	(12,514)
Amortization of Excess Deferred Income Taxes	(741)	(955)	(2,161)	(2,033)
North Dakota Wind Tax Credit Amortization, Net of Federal Tax	(220)	(150)	(530)	(505)
Corporate-Owned Life Insurance	(120)	241	(998)	1,383
Other, Net	660	(551)	(371)	(352)
Income Tax Expense / Effective Tax Rate	$ 24,727	$ 22,513	$ 58,093	$ 66,143
Increases (Decreases) in Tax from:
State Taxes on Income, Net of Federal Tax	5.00%	5.00%	5.00%	5.00%
Production Tax Credits (PTCs)	(4.40%)	(3.60%)	(4.90%)	(4.10%)
Amortization of Excess Deferred Income Taxes	(0.60%)	(0.90%)	(0.70%)	(0.70%)
North Dakota Wind Tax Credit Amortization, Net of Federal Tax	(0.20%)	(0.10%)	(0.20%)	(0.20%)
Corporate-Owned Life Insurance	(0.10%)	0.20%	(0.30%)	0.40%
Other, Net	0.50%	(0.50%)	(0.20%)	0.10%
Income Tax Expense / Effective Tax Rate	21.20%	21.10%	19.70%	21.50%

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