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Company's Liability for Environmental Obligations (Detail) - USD ($)
$ in Millions
 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Environmental Exit Cost [Line Items]    
Total Balance at Beginning of Period $ 2.0 $ 2.1
Additions 1.0 0.6
Less: Payments / Reductions 0.2 0.6
Total Balance at End of Period 2.8 2.1
Less: Current Portion 0.6 0.6
Noncurrent Balance at End of Period 2.2 1.5
Fitchburg Gas and Electric Light Company    
Environmental Exit Cost [Line Items]    
Total Balance at Beginning of Period   0.1
Less: Payments / Reductions   0.1
Northern Utilities Inc    
Environmental Exit Cost [Line Items]    
Total Balance at Beginning of Period 2.0 2.0
Additions 1.0 0.6
Less: Payments / Reductions 0.2 0.5
Total Balance at End of Period 2.8 2.1
Less: Current Portion 0.6 0.6
Noncurrent Balance at End of Period $ 2.2 $ 1.5