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Debt and Financing Arrangements - Additional Information (Detail) - USD ($)
 1 Months Ended 3 Months Ended 6 Months Ended 12 Months Ended
Dec. 18, 2019
Sep. 12, 2019
Jun. 30, 2020
Jul. 25, 2018
Jun. 30, 2020
Jun. 30, 2019
Mar. 31, 2019
Jun. 30, 2020
Jun. 30, 2019
Dec. 31, 2019
Jun. 16, 2020
Oct. 30, 2015
Line of Credit Facility [Line Items]                        
Weighted average interest rate on short term borrowings     1.50%   1.50% 3.60%   1.50% 3.60% 3.40%    
Gain on divestiture of business, net             $ 9,800,000          
Capital lease obligation, total capitalized cost     $ 549,000   $ 549,000 $ 400,000   $ 549,000 $ 400,000 $ 500,000    
Capital lease obligation, current     200,000   200,000 200,000   200,000 200,000 200,000    
Capital lease obligation, noncurrent     300,000   300,000 200,000   300,000 200,000 300,000    
Accounts Payable     24,700,000   24,700,000 21,900,000   24,700,000 21,900,000 37,600,000    
Guarantee outstanding     6,200,000   6,200,000     6,200,000        
Total rental expense under operating leases         400,000 300,000   800,000 700,000      
Operating lease obligations               800,000        
Net Utility Plant     1,147,600,000   1,147,600,000 1,053,500,000   1,147,600,000 1,053,500,000 1,111,500,000    
Other current operating lease obligation     1,300,000   1,300,000 1,000,000.0   1,300,000 1,000,000.0 1,200,000    
Other noncurrent operating lease obligation     $ 3,200,000   $ 3,200,000 2,600,000   $ 3,200,000 2,600,000 2,800,000    
Issuance of long-term debt $ 200,000 $ 200,000                    
Operating lease, weighted average remaining lease term     3 years 9 months 18 days   3 years 9 months 18 days     3 years 9 months 18 days        
Operating lease, weighted average discount rate percentage     4.90%   4.90%     4.90%        
Long-term Debt, Gross     $ 446,100,000   $ 446,100,000 $ 395,900,000   $ 446,100,000 $ 395,900,000 $ 460,500,000    
Credit Facility [Member] | Second Amended Credit Facility                        
Line of Credit Facility [Line Items]                        
Revolving credit facility       $ 120,000,000                
Sublimit for the issuance of standby letters of credit       $ 25,000,000                
Revolving credit facility termination date       Jul. 25, 2023                
Increase in borrowing limit       $ 50,000,000                
Credit Facility [Member] | London Interbank Offered Rate | Second Amended Credit Facility                        
Line of Credit Facility [Line Items]                        
Credit facility, daily fluctuating rate of interest       1.125%                
3.43% Senior Notes, Due December 18, 2029                        
Line of Credit Facility [Line Items]                        
Long-term debt, stated interest rate     3.43%   3.43% 3.43%   3.43% 3.43% 3.43%    
Long-term debt, maturity date               Dec. 18, 2029 Dec. 18, 2029 Dec. 18, 2029    
4.04% Senior Notes, Due September 12, 2049                        
Line of Credit Facility [Line Items]                        
Long-term debt, stated interest rate     4.04%   4.04% 4.04%   4.04% 4.04% 4.04%    
Long-term debt, maturity date               Sep. 12, 2049 Sep. 12, 2049 Sep. 12, 2049    
Revolving Credit Facility [Member]                        
Line of Credit Facility [Line Items]                        
Revolving credit facility     $ 120,000,000.0   $ 120,000,000.0 $ 120,000,000.0   $ 120,000,000.0 $ 120,000,000.0 $ 120,000,000.0    
Proceeds from lines of credit               121,600,000        
Repayments of lines of credit               $ 105,300,000        
Revolving Credit Facility [Member] | Credit Facility [Member]                        
Line of Credit Facility [Line Items]                        
Percentage of capitalization               The affirmative and negative covenants under the Credit Facility shall apply until the Credit Facility terminates and all amounts borrowed under the Credit Facility are paid in full (or with respect to letters of credit, they are cash collateralized). The only financial covenant in the Credit Facility provides that Funded Debt to Capitalization (as each term is defined in the Credit Facility) cannot exceed 65%, tested on a quarterly basis. At June 30, 2020, June 30, 2019 and December 31, 2019, the Company was in compliance with the covenants contained in the Credit Facility in effect on that date.        
Unitil Service Corp [Member]                        
Line of Credit Facility [Line Items]                        
Capital lease obligation, total capitalized cost                       $ 13,400,000
Northern Utilities Inc                        
Line of Credit Facility [Line Items]                        
Accounts Payable     100,000   100,000 100,000   $ 100,000 100,000 1,000,000.0    
Natural gas storage inventory     3,900,000   3,900,000 $ 5,000,000.0   $ 3,900,000 $ 5,000,000.0 6,500,000    
Total funded indebtedness as percentage of capitalization               65.00%        
Northern Utilities Inc | 4.04% Senior Notes, Due September 12, 2049                        
Line of Credit Facility [Line Items]                        
Long-term debt, aggregate principal amount   $ 40,000,000                    
Long-term debt, stated interest rate   4.04%       4.04%     4.04%      
Long-term debt, maturity date   Sep. 12, 2049             Sep. 12, 2049      
Long-term Debt, Gross     $ 40,000,000.0   40,000,000.0     $ 40,000,000.0   40,000,000.0    
Northern Utilities Inc | Senior Unsecured Notes Due 2040                        
Line of Credit Facility [Line Items]                        
Debt instrument year of maturity     2040                  
Northern Utilities Inc | Senior Unsecured Notes Due 2040 | Private Placement [Member]                        
Line of Credit Facility [Line Items]                        
Long-term debt, stated interest rate                     3.78%  
Long-term Debt, Gross                     $ 40,000,000  
Unitil Energy [Member] | First Mortgage Bonds Due 2040                        
Line of Credit Facility [Line Items]                        
Debt instrument year of maturity     2040                  
Fitchburg Gas and Electric Light Company | Senior Unsecured Notes Due 2040                        
Line of Credit Facility [Line Items]                        
Debt instrument year of maturity     2040                  
Fitchburg Gas and Electric Light Company | Senior Unsecured Notes Due 2040 | Private Placement [Member]                        
Line of Credit Facility [Line Items]                        
Long-term debt, stated interest rate                     3.78%  
Long-term Debt, Gross                     $ 27,500,000  
Unitil Corporation | 3.43% Senior Notes, Due December 18, 2029                        
Line of Credit Facility [Line Items]                        
Long-term debt, aggregate principal amount $ 30,000,000                      
Long-term debt, stated interest rate 3.43%                      
Long-term debt, maturity date Dec. 18, 2029                      
Northern Utilities And Fitchburg | Senior Unsecured Notes Due 2040 | Private Placement [Member]                        
Line of Credit Facility [Line Items]                        
Long-term Debt, Gross                     67,500,000  
Unitil Energy Systems Inc | Private Placement [Member]                        
Line of Credit Facility [Line Items]                        
Long-term Debt, Gross                     $ 95,000,000  
Unitil Energy Systems Inc | First Mortgage Bonds Due 2040 | Private Placement [Member]                        
Line of Credit Facility [Line Items]                        
Long-term debt, stated interest rate                     3.58%  
Long-term Debt, Gross                     $ 27,500,000  
Assets under Capital Leases [Member]                        
Line of Credit Facility [Line Items]                        
Net Utility Plant     $ 1,200,000   1,200,000 $ 1,500,000   1,200,000 $ 1,500,000 1,200,000    
Net Utility Plant, accumulated amortization     $ 600,000   $ 600,000 $ 1,000,000.0   $ 600,000 $ 1,000,000.0 $ 600,000