Energy Supply - Additional Information (Detail) - 12 months ended Dec. 31, 2023 Bcfe in Billions	Total	Total	BTU	Bcfe
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	6.3
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 | Bcfe				0.4
Percentage of power supply requirement		50.00%
Power supply contract duration	12 months
ISO New England markets
Gas and Oil Acreage [Line Items]
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