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Income Taxes - Effective Income Tax Rate (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Income Tax Disclosure [Abstract]
Income Taxes at Federal Statutory Rate	$ 21,426	$ 23,507	$ 37,341	$ 42,330
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	$ 5,104	$ 5,596	$ 8,893	$ 10,078
Production Tax Credits (PTCs)	(4,574)	(4,719)	(9,229)	(8,686)
Amortization of Excess Deferred Income Taxes	(622)	99	(1,420)	(1,078)
North Dakota Wind Tax Credit Amortization, Net of Federal Tax	(146)	(155)	(310)	(355)
Corporate-Owned Life Insurance	(460)	965	(878)	1,142
Other, Net	(666)	707	(1,032)	199
Income Tax Expense / Effective Tax Rate	$ 20,062	$ 26,000	$ 33,365	$ 43,630
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)	(4.50%)	(4.20%)	(5.20%)	(4.30%)
Amortization of Excess Deferred Income Taxes	(0.60%)	0.00%	(0.80%)	(0.50%)
North Dakota Wind Tax Credit Amortization, Net of Federal Tax	(0.10%)	(0.10%)	(0.20%)	(0.20%)
Corporate-Owned Life Insurance	(0.50%)	0.80%	(0.50%)	0.60%
Other, Net	(0.60%)	0.70%	(0.50%)	0.00%
Income Tax Expense / Effective Tax Rate	19.70%	23.20%	18.80%	21.60%

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