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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
Feb. 02, 2019
$ / shares
Dec. 31, 2018
USD ($)
$ / shares
Bcf
Sep. 30, 2018
$ / shares
Jun. 30, 2018
$ / shares
Mar. 31, 2018
$ / shares
Dec. 31, 2017
USD ($)
$ / shares
Bcf
Sep. 30, 2017
$ / shares
Jun. 30, 2017
$ / shares
Mar. 31, 2017
$ / shares
Dec. 31, 2018
USD ($)
Entity
Subsidiary
mi
$ / shares
Bcf
Dec. 31, 2017
USD ($)
$ / shares
Bcf
Dec. 31, 2016
USD ($)
$ / shares
Jan. 01, 2019
USD ($)
Significant Accounting Policies [Line Items]                          
Number of Subsidiaries | Entity                   3      
Length Of Pipeline | mi                   86      
Revenue sharing payments                   $ 1,000,000 $ 1,100,000 $ 1,000,000  
Other                   439,400,000 400,200,000 377,300,000  
Operation and Maintenance Expense                   $ 69,500,000 $ 64,500,000 $ 61,400,000  
Depreciation rate based on average depreciable property balance                   3.38% 3.45% 3.49%  
Common stock dividend paid per share | $ / shares   $ 0.365 $ 0.365 $ 0.365 $ 0.365 $ 0.360 $ 0.360 $ 0.360 $ 0.360 $ 1.46 $ 1.44 $ 1.42  
Common stock dividend per share, declared | $ / shares                   $ 1.46 $ 1.44 $ 1.42  
Cost of removal obligation   $ 90,700,000       $ 84,300,000       $ 90,700,000 $ 84,300,000    
Average interest rates                   2.70% 2.90% 2.18%  
Regulatory Assets   $ 140,300,000       $ 149,100,000       $ 140,300,000 $ 149,100,000    
Number of Natural Gas Storage Outstanding | Bcf   0.0       0.6       0.0 0.6    
Derivatives assets   $ 0       $ 0       $ 0 $ 0    
Derivatives liabilities   0       0       0 0    
Losses / (gains) recognized in regulatory assets / liabilities                   0 400,000    
Losses / (gains) reclassified into unaudited consolidated statement of earnings                   0 0    
Investments in trading securities   4,800,000       3,600,000       4,800,000 3,600,000    
ASU 2014-09                          
Significant Accounting Policies [Line Items]                          
Revenue sharing payments                     1,100,000 $ 1,000,000  
Operation and Maintenance Expense                     63,400,000 60,400,000  
Non Service                          
Significant Accounting Policies [Line Items]                          
Non-service cost net periodic benefit costs                   $ 5,500,000 5,700,000 $ 4,900,000  
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   3,500,000       2,900,000       $ 3,500,000 2,900,000    
Natural Gas Hedging Program                          
Significant Accounting Policies [Line Items]                          
Cash Deposits   0       $ 0       $ 0 $ 0    
Subsequent Event                          
Significant Accounting Policies [Line Items]                          
Common stock dividend per share, declared | $ / shares $ 1.48                        
Recognition of lease assets                         $ 4,200,000
Recognition of lease liabilities                         $ 4,200,000
Quarterly Dividends                          
Significant Accounting Policies [Line Items]                          
Common stock dividend paid per share | $ / shares                   $ 0.365 $ 0.360 $ 0.355  
Quarterly Dividends | Subsequent Event                          
Significant Accounting Policies [Line Items]                          
Common stock dividend per share, declared | $ / shares 0.37                        
Increase in dividend declared amount per share | $ / shares $ 0.005                        
Environmental and Rate Case Costs and Other Expenditures | Recovered over the next seven years                          
Significant Accounting Policies [Line Items]                          
Regulatory Assets   $ 6,000,000               $ 6,000,000      
Restatement Adjustment [Member] | ASU 2014-09                          
Significant Accounting Policies [Line Items]                          
Other                     $ (1,100,000) $ (1,000,000)  
Operation and Maintenance Expense                     $ (1,100,000) $ (1,000,000)  
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