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IPUC (Details) - Natural gas rate proceeding [Member] - IPUC [Member] - USD ($)
$ in Millions
Sep. 14, 2017
Aug. 17, 2017
Apr. 28, 2017
Jan. 17, 2017
Aug. 12, 2016
Public Utilities, General Disclosures [Line Items]          
Public utilities, requested rate increase (decrease), amount   $ 1.2     $ 10.2
Public utilities, requested rate increase (decrease), percentage   1.36%     4.10%
Public utilities, requested rate increase (decrease), amended, amount       $ 9.4  
Public utilities, approved rate increase (decrease), amount $ 6.7   $ 4.1    
Public utilities, approved rate increase (decrease), percentage     1.60%    
Public utilities, approved return on equity, percentage     9.50%