| Document and Entity Information | Dec. 17, 2019 |
| Cover [Abstract] | |
| Entity Registrant Name | UNITIL CORP |
| Amendment Flag | false |
| Entity Central Index Key | 0000755001 |
| Document Type | 8-K |
| Document Period End Date | Dec. 17, 2019 |
| Entity Incorporation State Country Code | NH |
| Entity File Number | 1-8858 |
| Entity Tax Identification Number | 02-0381573 |
| Entity Address, Address Line One | 6 Liberty Lane West |
| Entity Address, City or Town | Hampton |
| Entity Address, State or Province | NH |
| Entity Address, Postal Zip Code | 03842-1720 |
| City Area Code | (603) |
| Local Phone Number | 772-0775 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock |
| Trading Symbol | UTL |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
