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MTPSC (Details 2) - MTPSC - USD ($)
Apr. 24, 2019
Mar. 07, 2019
Dec. 21, 2018
Sep. 28, 2018
Electric rate proceeding        
Public Utilities, General Disclosures [Line Items]        
Public utilities, requested rate increase (decrease), amount       $ 11,900,000
Public utilities, requested rate increase (decrease), percentage       18.90%
Public utilities, interim rate increase (decrease), amount   $ 7,900,000    
Public utilities, interim rate increase (decrease), percentage   12.80%    
Subsequent Event [Member] | Electric rate proceeding        
Public Utilities, General Disclosures [Line Items]        
Public utilities, requested rate increase (decrease), amount $ 9,000,000.0      
Subsequent Event [Member] | Future Electric Rate Proceeding [Member]        
Public Utilities, General Disclosures [Line Items]        
Public utilities, requested rate increase (decrease), amount $ 300,000      
Tax Cuts and Jobs Act [Member] | Electric rate proceeding        
Public Utilities, General Disclosures [Line Items]        
Public Utilities, Approved One-Time Increase (Decrease) to Customers, Amount     $ (1,500,000)