XML 35 R52.htm IDEA: XBRL DOCUMENT v2.4.0.8
Energy Supply - Additional Information (Detail)
 1 Months Ended 12 Months Ended
Jul. 31, 2012
Dec. 31, 2013
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   14,057
Percentage of power supply requirement   50.00%
Power supply contract duration   12 months
Fitchburg Gas and Electric Light Company | Maximum
    
Gas and Oil Acreage [Line Items]    
Purchases of natural gas, contract duration   1 year
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   100,000
Natural gas, underground storage   3.4
Northern Utilities Inc | Maximum
    
Gas and Oil Acreage [Line Items]    
Purchases of natural gas, contract duration   1 year
Unitil Energy Systems Inc
    
Gas and Oil Acreage [Line Items]    
Percentage of power supply requirement 100.00% 100.00%
Power supply contract duration 6 months 12 months