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Energy Supply - Additional Information (Detail) | 12 Months Ended
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Dec. 31, 2017 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 | 3,600,000,000
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 | 14,057
Natural gas, underground storage | Bcf | 0.33
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

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