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Company's Liability for Environmental Obligations (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Environmental Exit Cost [Line Items]    
Total Balance at Beginning of Period $ 2.0 $ 2.1
Additions 0.9 0.3
Less: Payments / Reductions 0.2 0.4
Total Balance at End of Period 2.7 2.0
Less: Current Portion 0.6 0.6
Noncurrent Balance at End of Period 2.1 1.4
Fitchburg Gas and Electric Light Company    
Environmental Exit Cost [Line Items]    
Total Balance at Beginning of Period 0.0 0.1
Less: Payments / Reductions 0.0 0.1
Total Balance at End of Period 0.0
Less: Current Portion 0.0  
Noncurrent Balance at End of Period 0.0  
Northern Utilities Inc    
Environmental Exit Cost [Line Items]    
Total Balance at Beginning of Period 2.0 2.0
Additions 0.9 0.3
Less: Payments / Reductions 0.2 0.3
Total Balance at End of Period 2.7 2.0
Less: Current Portion 0.6 0.6
Noncurrent Balance at End of Period $ 2.1 $ 1.4