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Debt and Financing Arrangements - Additional Information (Detail) - USD ($) |  | 1 Months Ended | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Dec. 18, 2020 | Jul. 25, 2018 | Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019 | Sep. 15, 2020 | Dec. 18, 2019 | Sep. 12, 2019 | Oct. 30, 2015
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |
Issuance of long-term debt | $ 100,000 |  |  |  |  | $ 500,000 | $ 200,000 | $ 200,000 |
Weighted average interest rate on short term borrowings |  |  | 1.20% | 1.70% | 3.40% |  |  |  |
Guarantee outstanding |  |  | $ 700,000 |  |  |  |  |  |
Capital lease obligation, current |  |  | 100,000 |  |  |  |  |  |
Accounts Payable |  |  | 52,400,000 | $ 33,200,000 |  |  |  |  |
Total rental expense under operating leases |  |  | 1,900,000 | 1,800,000 | $ 1,400,000 |  |  |  |
Operating lease obligations |  |  | 1,900,000 | 1,800,000 |  |  |  |  |
Net Utility Plant |  |  | 1,257,200,000 | $ 1,193,200,000 |  |  |  |  |
Other current operating lease obligation |  |  | $ 1,600,000 |  |  |  |  |  |
Operating lease, weighted average remaining lease term |  |  | 3 years 6 months | 3 years 9 months 18 days |  |  |  |  |
Operating lease, weighted average discount rate percentage |  |  | 3.90% | 4.40% |  |  |  |  |
Restriction on retained earnings for dividend payments |  |  | Therefore, there were no restrictions on the Company’s Retained Earnings at December 31, 2021 for the payment of dividends. |  |  |  |  |  |
Amount available for the payment of dividends |  |  | $ 166,900,000 |  |  |  |  |  |
Retained Earnings |  |  | 116,200,000 | $ 103,700,000 |  |  |  |  |
Long term debt repayments |  |  | $ 25,800,000 | $ 24,800,000 | 18,800,000 |  |  |  |
Operating Lease, Liability, Current, Statement of Financial Position [Extensible Enumeration] |  |  | Other current operating lease obligation |  |  |  |  |  |
Lease Obligations [Member] |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |
Capital lease obligation, noncurrent |  |  | $ 200,000 |  |  |  |  |  |
Other noncurrent operating lease obligation |  |  | 3,100,000 |  |  |  |  |  |
Unitil Energy, Fitchburg, Northern Utilities and Granite State |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |
Amount available for the payment of dividends |  |  | $ 358,700,000 |  |  |  |  |  |
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% |  |  |  |  |
Long-term debt, maturity date |  |  | Dec. 18, 2029 | Dec. 18, 2029 |  |  |  |  |
Bonds [Member] |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |
Long term debt repayments |  |  | $ 25,800,000 | $ 24,800,000 | $ 18,800,000 |  |  |  |
Debt repayment, 2022 |  |  | 8,400,000 |  |  |  |  |  |
Debt repayment, 2023 |  |  | 6,900,000 |  |  |  |  |  |
Debt repayment, 2024 |  |  | 6,900,000 |  |  |  |  |  |
Debt repayment, 2025 |  |  | 5,000,000.0 |  |  |  |  |  |
Debt repayment, 2026 |  |  | 38,000,000.0 |  |  |  |  |  |
Debt repayment, Thereafter |  |  | 444,400,000 |  |  |  |  |  |
Revolving Credit Facility [Member] |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |
Revolving credit facility |  |  | 120,000,000.0 | 120,000,000.0 |  |  |  |  |
Proceeds from lines of credit |  |  | 239,100,000 | 248,900,000 |  |  |  |  |
Repayments of lines of credit |  |  | $ 229,700,000 | 252,800,000 |  |  |  |  |
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 to Unitil 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 Unitil’s Funded Debt to Capitalization (as each term is defined in the Credit Facility) cannot exceed 65%, tested on a quarterly basis. |  |  |  |  |  |
Accounts Payable |  |  |  | 1,000,000.0 |  |  |  |  |
Natural gas storage inventory |  |  | $ 8,300,000 | $ 5,400,000 |  |  |  |  |
Unitil Service Corp |  |  |  |  |  |  |  |  |
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 |  |  | $ 1,600,000 |  |  |  |  |  |
Long-term debt, aggregate principal amount |  |  |  |  |  | 40,000,000 |  |  |
Long-term debt, stated interest rate |  |  |  |  |  |  |  | 4.04% |
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% |  |  |  |  |
Long-term debt, maturity date |  |  | Sep. 12, 2049 | Sep. 12, 2049 |  |  |  |  |
Granite State Gas Transmission Inc |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |
Total funded indebtedness as percentage of capitalization |  |  | 65.00% |  |  |  |  |  |
Fitchburg Gas and Electric Light Company |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |
Total funded indebtedness as percentage of capitalization |  |  | 65.00% |  |  |  |  |  |
Fitchburg Gas and Electric Light Company | 3.78% Senior Notes, Due September 15, 2040 |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |
Long-term debt, aggregate principal amount |  |  |  |  |  | $ 27,500,000 |  |  |
Long-term debt, stated interest rate |  |  |  |  |  | 3.78% |  |  |
Unitil Corporation | Maximum [Member] |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |
Total funded indebtedness as percentage of capitalization |  |  | 70.00% |  |  |  |  |  |
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% |  |
Northern Utilities And Fitchburg | 3.78% Senior Notes, Due September 15, 2040 |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |
Long-term debt, stated interest rate |  |  |  |  |  | 3.78% |  |  |
Unitil Energy Systems Inc | 3.58% Mortgage Bonds, Due September 12, 2040 |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |
Long-term debt, aggregate principal amount |  |  |  |  |  | $ 27,500,000 |  |  |
Long-term debt, stated interest rate |  |  |  |  |  | 3.58% |  |  |
Unitil Reality Corp | 2.64% Senior Secured Notes, Due December 18, 2030 |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |
Long-term debt, aggregate principal amount | $ 4,700,000 |  |  |  |  |  |  |  |
Long-term debt, stated interest rate | 2.64% |  |  |  |  |  |  |  |
Long-term debt, maturity date | Dec. 18, 2030 |  |  |  |  |  |  |  |
Assets under Capital Leases [Member] |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |
Net Utility Plant |  |  | $ 700,000 | $ 1,000,000.0 |  |  |  |  |
Net Utility Plant, accumulated amortization |  |  | $ 300,000 | $ 500,000 |  |  |  |  |

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