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Cover Page - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Feb. 09, 2024
Jun. 30, 2023
Document Information [Line Items]      
Document Type 10-K    
Document Annual Report true    
Document Transition Report false    
Entity Interactive Data Current Yes    
Amendment Flag false    
Document Period End Date Dec. 31, 2023    
Document Fiscal Year Focus 2023    
Document Fiscal Period Focus FY    
Entity Registrant Name UNITIL CORPORATION    
Entity Central Index Key 0000755001    
Entity File Number 1-8858    
Entity Tax Identification Number 02-0381573    
Entity Incorporation, State or Country Code NH    
Current Fiscal Year End Date --12-31    
Entity Current Reporting Status Yes    
Document Financial Statement Error Correction [Flag] false    
Entity Shell Company false    
Entity Filer Category Large Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
Entity Address, Address Line One 6 Liberty Lane West    
Entity Address, City or Town Hampton    
Entity Address, Postal Zip Code 03842-1720    
Entity Address, State or Province NH    
City Area Code 603    
Local Phone Number 772-0775    
Trading Symbol UTL    
Security Exchange Name NYSE    
Entity Common Stock, Shares Outstanding   16,163,365  
Title of 12(b) Security Common Stock    
Entity Public Float     $ 802,806,380
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
ICFR Auditor Attestation Flag true    
Documents Incorporated by Reference [Text Block]

Portions of the Proxy Statement relating to the Annual Meeting of Shareholders to be held on May 1, 2024 are incorporated by reference into Part III of this Report.

   
Auditor Name Deloitte & Touche LLP    
Auditor Firm ID 34    
Auditor Location Boston, MA