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NDPSC (Details 4) - NDPSC [Member] - USD ($)
$ in Millions
Feb. 14, 2018
Sep. 06, 2017
Jul. 21, 2017
Jun. 30, 2017
Advance determination of prudence [Member]        
Public Utilities, General Disclosures [Line Items]        
Estimated project cost       $ 85.0
Natural gas rate proceeding [Member]        
Public Utilities, General Disclosures [Line Items]        
Public utilities, requested rate increase (decrease), amount     $ 5.9  
Public utilities, requested rate increase (decrease), percentage     5.40%  
Public utilities, interim rate increase (decrease), amount   $ 4.6    
Public utilities, interim rate increase (decrease), percentage   4.20%    
Subsequent event [Member]        
Public Utilities, General Disclosures [Line Items]        
Public utilities interim rate increase (decrease), revised $ 2.7