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Income Tax Expense (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Current Income Tax Expense [Abstract]      
Federal [1] $ 0.0 $ 0.0  
State [1] 0.7 0.1  
Total Current Income Tax Expense 0.7 0.1  
Deferred Income Tax Expense (Benefit) [Abstract]      
Federal (6.8) [2] 7.3 [3]  
State 2.6 5.9  
Investment Tax Credit Amortization (0.2) (0.2)  
Total Deferred Income Tax Expense (Benefit) (4.4) 13.0  
Total Income Tax Expense (Benefit) (3.7) 13.1  
Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense [Abstract]      
Income Before Non-Controlling Interest and Income Taxes $ 47.3 $ 62.1  
Statutory Federal Income Tax Rate 21.00% 35.00% 35.00%
Income Taxes Computed at Statutory Federal Rate $ 9.9 $ 21.7  
Increase (Decrease) in Tax Due to: [Abstract]      
State Income Taxes – Net of Federal Income Tax Benefit 2.6 3.9  
Production Tax Credits (14.4) (13.0)  
Regulatory Differences for Utility Plant (2.5) 0.1  
Other 0.7 0.4  
Total Income Tax Expense (Benefit) $ (3.7) $ 13.1  
Effective Tax Rate (7.80%) 21.10%  
Uncertain Tax Positions [Abstract]      
Gross Unrecognized Tax Benefits $ 1.7   $ 1.7
Gross Unrecognized Tax Benefits That Would Favorably Impact Effective Income Tax Rate $ 0.8    
[1] For the three months ended March 31, 2018, and 2017, the federal and state current tax expense was minimal due to NOLs which resulted from the bonus depreciation provisions of the Protecting Americans from Tax Hikes Act of 2015, the Tax Increase Prevention Act of 2014 and the American Taxpayer Relief Act of 2012.
[2] We have recorded the impact of the federal income tax rate change in 2018 due to the TCJA for Minnesota Power and SWL&P as regulatory liabilities and a reduction in revenue as the benefits of the TCJA are deferred pending the outcome of regulatory proceedings with the MPUC and PSCW. (See Tax Cuts and Jobs Act of 2017.)
[3] For the three months ended March 31, 2018, the federal tax benefit is primarily due to the reduction of the federal statutory tax rate from 35 percent to 21 percent enacted as part of the TCJA, and production tax credits.