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Summary of Significant Accounting Policies - Additional Information (Detail)
$ / shares in Units, Bcf in Billions
 1 Months Ended 3 Months Ended 12 Months Ended
Jan. 28, 2016
$ / shares
Dec. 31, 2015
USD ($)
$ / shares
Bcf
Sep. 30, 2015
$ / shares
Jun. 30, 2015
$ / shares
Mar. 31, 2015
$ / shares
Dec. 31, 2014
USD ($)
$ / shares
Bcf
Sep. 30, 2014
$ / shares
Jun. 30, 2014
$ / shares
Mar. 31, 2014
$ / shares
Dec. 31, 2015
USD ($)
Entity
Subsidiary
mi
$ / shares
Bcf
Dec. 31, 2014
USD ($)
$ / shares
Bcf
Dec. 31, 2013
$ / shares
Significant Accounting Policies [Line Items]                        
Number of Subsidiaries | Entity                   3    
Length Of Pipeline | mi                   86    
Depreciation rate based on average depreciable property balance                   3.57% 3.56% 3.59%
Common stock dividend paid per share | $ / shares   $ 0.350 $ 0.350 $ 0.350 $ 0.350 $ 0.345 $ 0.345 $ 0.345 $ 0.345 $ 1.40 $ 1.38 $ 1.38
Common stock dividend per share, declared | $ / shares                   $ 1.40 $ 1.38 $ 1.38
Average interest rates                   2.32% 1.56% 1.92%
Cost of removal obligation   $ 70,100,000       $ 63,800,000       $ 70,100,000 $ 63,800,000  
Regulatory Assets   $ 126,400,000       $ 145,400,000       $ 126,400,000 $ 145,400,000  
Number of Natural Gas Storage Outstanding | Bcf   2.5       2.4       2.5 2.4  
Investments in trading securities   $ 700,000       $ 0       $ 700,000 $ 0  
Intangible Assets, Purchase Adjustments                   $ 7,200,000    
Plant                        
Significant Accounting Policies [Line Items]                        
Intangible Asset Amortization Period                   10 years    
Bargain Purchase Adjustment                        
Significant Accounting Policies [Line Items]                        
Intangible Asset Amortization Period                   3 years    
Annual Electric Sales Volume                        
Significant Accounting Policies [Line Items]                        
Percentage of total sales volumes revenue subject to RDM                   27.00%    
Annual Natural Gas Sales Volume                        
Significant Accounting Policies [Line Items]                        
Percentage of total sales volumes revenue subject to RDM                   11.00%    
Maximum                        
Significant Accounting Policies [Line Items]                        
Cash equivalents maturity period                   3 months    
ISO-NE Obligations                        
Significant Accounting Policies [Line Items]                        
Cash Deposits   2,300,000       6,300,000       $ 2,300,000 6,300,000  
Subsequent Event                        
Significant Accounting Policies [Line Items]                        
Common stock dividend per share, declared | $ / shares $ 1.42                      
Natural Gas Hedging Program                        
Significant Accounting Policies [Line Items]                        
Cash Deposits   0       0       $ 0 $ 0  
Quarterly Dividends                        
Significant Accounting Policies [Line Items]                        
Common stock dividend paid per share | $ / shares                   $ 0.350 $ 0.345 $ 0.345
Quarterly Dividends | Subsequent Event                        
Significant Accounting Policies [Line Items]                        
Common stock dividend per share, declared | $ / shares 0.3550                      
Increase in dividend declared amount per share | $ / shares $ 0.005                      
Deferred Storm Charges                        
Significant Accounting Policies [Line Items]                        
Regulatory Assets   15,400,000       $ 21,200,000       $ 15,400,000 $ 21,200,000  
Deferred Storm Charges | Recovered over the next two and a half years                        
Significant Accounting Policies [Line Items]                        
Regulatory Assets   6,600,000               6,600,000    
Rate Case Costs and Other Expenditures | Recovered over the next seven years                        
Significant Accounting Policies [Line Items]                        
Regulatory Assets   $ 7,400,000               $ 7,400,000    
Utilities                        
Significant Accounting Policies [Line Items]                        
Number of Subsidiaries | Subsidiary                   3    
Unitil Service; Unitil Realty; and Unitil Resources                        
Significant Accounting Policies [Line Items]                        
Number of Subsidiaries | Subsidiary                   3