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Energy Supply - Additional Information (Detail)
 12 Months Ended
Dec. 31, 2019
Bcf
MMBTU
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 | MMBTU 115,000,000,000
Natural gas, underground storage | Bcf 4,300,000,000
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 | MMBTU 14,439
Natural gas, underground storage | Bcf 0.43
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