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Note 14 - Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred Tax Assets
Benefit Liabilities	$ 36,246	$ 33,967
Retirement Benefits Liabilities	36,206	32,664
Regulatory Tax Liability	35,700	33,228
North Dakota Wind Tax Credits	31,611	32,570
Cost of Removal	25,604	21,787
Federal PTCs	20,017	32,101
Differences Related to Property	6,979	6,842
Lease Liability	5,733	0
Vacation Accrual	1,884	1,919
Net Operating Loss Carryforward	1,860	2,489
Investment Tax Credits	408	449
Other	344	3,218
Valuation Allowance	(800)	(600)
Total Deferred Tax Assets	201,792	200,634
Deferred Tax Liabilities
Differences Related to Property	(268,495)	(261,396)
Retirement Benefits Regulatory Asset	(36,206)	(32,664)
Excess Tax over Book Pension	(17,556)	(15,145)
Right of Use Asset	(5,705)	0
North Dakota Wind Tax Credits	(3,126)	(4,386)
Impact of State Net Operating Losses on Federal Taxes	(385)	(523)
Other	(2,260)	(7,496)
Total Deferred Tax Liabilities	(333,733)	(321,610)
Deferred Income Taxes	$ (131,941)	$ (120,976)

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