XML 22 R1.htm IDEA: XBRL DOCUMENT v3.19.3.a.u2
Cover Page - USD ($)
 12 Months Ended
Dec. 31, 2019
Jan. 27, 2020
Jun. 30, 2019
Document Information [Line Items]      
Entity Interactive Data Current Yes    
Document Type 10-K    
Document Transition Report false    
Amendment Flag false    
Document Period End Date Dec. 31, 2019    
Document Fiscal Year Focus 2019    
Document Fiscal Period Focus FY    
Entity Registrant Name UNITIL CORP    
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   14,930,967  
Title of 12(b) Security Common Stock    
Document Annual Report true    
Entity Public Float     $ 880,678,701
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No