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Cover Page - USD ($)
 12 Months Ended
Dec. 31, 2021
Jan. 28, 2022
Jun. 30, 2021
Document Information [Line Items]      
Entity Interactive Data Current Yes    
Document Transition Report false    
Document Type 10-K    
Amendment Flag false    
Document Period End Date Dec. 31, 2021    
Document Fiscal Year Focus 2021    
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    
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   15,978,791  
Title of 12(b) Security Common Stock    
Document Annual Report true    
Entity Public Float     $ 785,923,009
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
ICFR Auditor Attestation Flag true    
Auditor Name Deloitte & Touche LLP    
Auditor Firm ID 34    
Auditor Location Boston, MA