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Energy Supply - Additional Information (Detail)
 12 Months Ended 1 Months Ended
Dec. 31, 2012
Fitchburg Gas and Electric Light Company
MMBTU
Dec. 31, 2012
Fitchburg Gas and Electric Light Company
Maximum
Dec. 31, 2012
Northern Utilities Inc
MMBTU
Bcf
Dec. 31, 2012
Northern Utilities Inc
Maximum
Jul. 31, 2012
Unitil Energy Systems Inc
Gas and Oil Acreage [Line Items]          
Purchases of natural gas, contract duration   1 year   1 year  
Natural gas available under firm contract per day of year-round and seasonal transportation and underground storage capacity to distribution facilities 14,057   100,000    
Natural gas, underground storage     3.4    
Power supply contract duration 12 months       6 months
Percentage of power supply requirement 50.00%       100.00%