XML 74 R55.htm IDEA: XBRL DOCUMENT v3.3.1.900
Commitments and Contingencies - Additional Information (Detail) - USD ($)
 3 Months Ended 6 Months Ended 12 Months Ended
Jun. 16, 2015
Apr. 15, 2015
Nov. 26, 2014
May. 30, 2014
Apr. 30, 2014
Apr. 21, 2014
Dec. 27, 2013
Apr. 25, 2013
Dec. 31, 2015
Jun. 30, 2015
Dec. 31, 2015
Dec. 31, 2014
Regulatory Assets [Line Items]                        
Regulatory Assets, Remaining Balance                 $ 1,900,000   $ 1,900,000  
Accrued Revenue                 38,400,000   38,400,000 $ 48,500,000
Regulatory Assets                 99,600,000   99,600,000 107,600,000
Current Portion of Regulatory Liabilities                 15,600,000   15,600,000 8,700,000
Regulatory Liabilities - noncurrent                 8,100,000   $ 8,100,000 $ 100,000
Maine | Environmental Restoration Costs                        
Regulatory Assets [Line Items]                        
Amortization period for environmental costs                     5 years  
New Hampshire | Environmental Restoration Costs                        
Regulatory Assets [Line Items]                        
Amortization period for environmental costs                     7 years  
Other Restructuring                        
Regulatory Assets [Line Items]                        
Accrued Revenue                 300,000   $ 300,000  
Regulatory Assets                 1,600,000   $ 1,600,000  
Cost recovery period, years                     6 years  
Environmental Matters                        
Regulatory Assets [Line Items]                        
Income (loss) incurred in excess of recorded amount                     $ 0  
Northern Utilities Inc                        
Regulatory Assets [Line Items]                        
Increase in annual revenue           $ 1,400,000            
Amendment effective date                     May 01, 2014  
TIRA initial term                     Four years  
First annual TIRA Adjustment, effective date             May 01, 2014          
Pipeline refund received   $ 22,000,000                    
Current Portion of Regulatory Liabilities                 7,600,000   $ 7,600,000  
Regulatory Liabilities - noncurrent                 8,100,000   $ 8,100,000  
Northern Utilities Inc | Maine                        
Regulatory Assets [Line Items]                        
Increase in annual revenue             $ 3,800,000          
Amendment effective date                     Jan. 01, 2014  
Grace period to receive refund                     3 years  
Date to receive a refund as one-time lump sum payment                     Oct. 05, 2015  
Net gain on sale of property                 900,000      
Northern Utilities Inc | New Hampshire                        
Regulatory Assets [Line Items]                        
Increase in annual revenue           1,800,000            
Amendment effective date                     May 01, 2015  
Grace period to receive refund                     3 years  
Northern Utilities Inc | New Hampshire | Natural Gas Distribution                        
Regulatory Assets [Line Items]                        
Increase in annual revenue           $ 4,600,000            
Unitil Energy Systems Inc                        
Regulatory Assets [Line Items]                        
Amendment effective date                     May 01, 2014  
Date to receive a refund as one-time lump sum payment         May 01, 2016              
Adjustment to recover the increased spending for vegetation management program and reliability enhancement program and a proposed storm resiliency program         $ 1,500,000              
Interest rate allowed by regulators on the unrecovered amount of storm costs which have been approved for recovery through rate adjustments               4.52%        
Construction expenditures     $ 400,000                  
Recovery of damage of electrical system     1,700,000                  
Unitil Energy Systems Inc | Storm Costs                        
Regulatory Assets [Line Items]                        
Cost recovery period, years               5 years        
Recovery amount               $ 2,300,000        
Storm expenditures     2,100,000                  
Granite State Gas Transmission Inc | Gas Transportation and Storage                        
Regulatory Assets [Line Items]                        
Increase in annual revenue                   $ 400,000    
Amendment effective date                   Aug. 01, 2015    
Fitchburg Gas and Electric Light Company                        
Regulatory Assets [Line Items]                        
Cost recovery period, years       5 years                
Increase in annual revenue       $ 5,600,000                
Amendment effective date                     May 01, 2016  
Percentage of approved return on equity       9.70%                
Percentage of approved common equity ratio       48.00%                
Annual funding amount approved       $ 500,000                
Recovery amount       $ 900,000                
Storm expenditures     200,000                  
Construction expenditures     100,000                  
Fitchburg Gas and Electric Light Company | Amendment Agreement [Member]                        
Regulatory Assets [Line Items]                        
Amendment effective date                     Jun. 01, 2014  
Fitchburg Gas and Electric Light Company | Cumulative capital investments for April 30, 2015                        
Regulatory Assets [Line Items]                        
Recovery amount                 300,000   $ 300,000  
Fitchburg Gas and Electric Light Company | Cumulative capital investments for 2015 and 2016                        
Regulatory Assets [Line Items]                        
Recovery amount                 $ 900,000   $ 900,000  
Fitchburg Gas and Electric Light Company | Electric base                        
Regulatory Assets [Line Items]                        
Increase in annual revenue $ 3,800,000                      
Percentage of approved return on equity 5.60%                      
Fitchburg Gas and Electric Light Company | Gas base                        
Regulatory Assets [Line Items]                        
Increase in annual revenue $ 3,000,000                      
Percentage of approved return on equity 8.30%                      
Fitchburg Gas and Electric Light Company | Storm Costs                        
Regulatory Assets [Line Items]                        
Cost recovery period, years       3 years                
Deferred emergency storm repair costs incurred       $ 5,000,000                
Storm expenditures     $ 300,000 $ 5,000,000