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Company's Liability for Environmental Obligations (Detail) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Dec. 31, 2020
Environmental Exit Cost [Line Items]      
Total Balance at Beginning of Period $ 2.1 $ 2.7  
Additions 0.1 0.0  
Less: Payments / Reductions 0.1 0.5  
Total Balance at End of Period 2.1 2.2  
Less: Current Portion 0.3 0.3 $ 0.3
Noncurrent Balance at End of Period 1.8 1.9 $ 1.8
Fitchburg Gas and Electric Light Company      
Environmental Exit Cost [Line Items]      
Total Balance at Beginning of Period 0.1 0.0  
Additions 0.0 0.0  
Less: Payments / Reductions 0.0 0.0  
Total Balance at End of Period 0.1 0.0  
Less: Current Portion 0.1 0.0  
Noncurrent Balance at End of Period 0.0 0.0  
Northern Utilities Inc      
Environmental Exit Cost [Line Items]      
Total Balance at Beginning of Period 2.0 2.7  
Additions 0.1 0.0  
Less: Payments / Reductions 0.1 0.5  
Total Balance at End of Period 2.0 2.2  
Less: Current Portion 0.2 0.3  
Noncurrent Balance at End of Period $ 1.8 $ 1.9