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Company's Liability for Environmental Obligations (Detail) - USD ($)
$ in Millions
 9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Dec. 31, 2019
Environmental Exit Cost [Line Items]      
Total Balance at Beginning of Period $ 2.7 $ 2.0  
Additions 0.1 1.0  
Less: Payments / Reductions 0.6 0.2  
Total Balance at End of Period 2.2 2.8  
Less: Current Portion 0.3 0.6 $ 0.6
Noncurrent Balance at End of Period 1.9 2.2 $ 2.1
Fitchburg Gas and Electric Light Company      
Environmental Exit Cost [Line Items]      
Total Balance at Beginning of Period  
Additions  
Less: Payments / Reductions  
Total Balance at End of Period  
Less: Current Portion  
Noncurrent Balance at End of Period  
Northern Utilities Inc      
Environmental Exit Cost [Line Items]      
Total Balance at Beginning of Period 2.7 2.0  
Additions 0.1 1.0  
Less: Payments / Reductions 0.6 0.2  
Total Balance at End of Period 2.2 2.8  
Less: Current Portion 0.3 0.6  
Noncurrent Balance at End of Period $ 1.9 $ 2.2