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Energy Supply - Additional Information (Detail) - 12 months ended Dec. 31, 2022
Bcf in Billions
Total
Total
BTU
Bcf
Northern Utilities Inc        
Gas and Oil Acreage [Line Items]        
Natural gas available under firm contract per day of year-round and seasonal transportation and underground storage capacity to distribution facilities     85,500,000,000  
Natural gas, underground storage     44,000 4.3
Northern Utilities Inc | Maximum        
Gas and Oil Acreage [Line Items]        
Purchases of natural gas, contract duration 1 year      
Fitchburg Gas and Electric Light Company        
Gas and Oil Acreage [Line Items]        
Natural gas available under firm contract per day of year-round and seasonal transportation and underground storage capacity to distribution facilities     14,439  
Natural gas, underground storage | Bcf       0.4
Percentage of power supply requirement   50.00%    
Power supply contract duration 12 months      
Unitil Energy Systems Inc        
Gas and Oil Acreage [Line Items]        
Percentage of power supply requirement   100.00%    
Power supply contract duration 6 months