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Significant Segment Financial Data (Detail) (USD $)
In Millions, unless otherwise specified
3 Months Ended 6 Months Ended
Jun. 30, 2012
Jun. 30, 2011
Jun. 30, 2012
Jun. 30, 2011
Dec. 31, 2011
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]          
Revenues $ 68.8 $ 69.5 $ 183.0 $ 184.9  
Segment Profit (Loss) (0.4) (0.8) 8.6 7.9  
Capital Expenditures 14.8 14.3 23.9 25.1  
Segment Assets 768.3 727.6 768.3 727.6 800.2
Electricity
         
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]          
Revenues 44.8 42.9 93.5 91.1  
Segment Profit (Loss) 1.4 1.6 2.6 3.3  
Capital Expenditures 4.1 4.5 9.4 9.8  
Segment Assets 377.7 364.5 377.7 364.5  
Gas Segment
         
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]          
Revenues 22.7 25.2 86.9 91.1  
Segment Profit (Loss) (2.2) (3.0) 5.4 3.8  
Capital Expenditures 10.0 8.8 13.1 14.0  
Segment Assets 380.2 350.5 380.2 350.5  
All Other Segments
         
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]          
Segment Profit (Loss) 0.1 0.1   (0.1)  
Capital Expenditures 0.7 1.0 1.4 1.3  
Segment Assets 3.4 6.0 3.4 6.0  
Unregulated Operation
         
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]          
Revenues 1.3 1.4 2.6 2.7  
Segment Profit (Loss) 0.3 0.5 0.6 0.9  
Segment Assets $ 7.0 $ 6.6 $ 7.0 $ 6.6