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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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