Summary of Significant Accounting Policies - Additional Information (Detail) $ / shares in Units, $ in Millions	1 Months Ended	12 Months Ended
	Jan. 31, 2022 $ / shares	Dec. 31, 2021 USD ($) Subsidiary mi $ / shares	Dec. 31, 2020 USD ($) $ / shares	Dec. 31, 2019 USD ($) $ / shares	Dec. 31, 2018 USD ($)
Significant Accounting Policies [Line Items]
Length Of Pipeline | mi		86
Gain on divestiture of business, pretax				$ 13.4
Gain on divestiture of business, net				9.8
Provision for income taxes				3.6
Cost of removal obligation		$ 107.5	$ 105.2
Regulatory assets		156.3	164.7
Investments in trading securities		$ 5.7	5.7
Number of Subsidiaries | Subsidiary		3
Allowance for doubtful accounts		$ 3.3	$ 3.3	$ 1.0	$ 1.3
Depreciation rate based on average depreciable property balance		3.29%	3.34%	3.41%
Common stock dividend per share, declared | $ / shares		$ 1.52	$ 1.50	$ 1.48
Increase in dividend declared amount per share | $ / shares		$ 1.52
Inventory of materials and supplies		$ 1.0	$ 0.6
Average Interest Rate On Debt		1.71%	3.12%	3.90%
Other Deferred Charges [Member]
Significant Accounting Policies [Line Items]
Regulatory assets		$ 15.3	$ 14.2
Materials And Supplies [Member]
Significant Accounting Policies [Line Items]
Inventory of materials and supplies		8.6	8.5
Subsequent Event [Member]
Significant Accounting Policies [Line Items]
Increase in dividend declared amount per share | $ / shares	$ 1.56
Unbilled Revenues
Significant Accounting Policies [Line Items]
Allowance for doubtful accounts		0.2	0.2
Accounts Receivable
Significant Accounting Policies [Line Items]
Allowance for doubtful accounts		$ 3.1	3.1
Utilities
Significant Accounting Policies [Line Items]
Number of Subsidiaries | Subsidiary		3
Unitil Service; Unitil Realty; and Unitil Resources
Significant Accounting Policies [Line Items]
Number of Subsidiaries | Subsidiary		3
Fitchburg Gas and Electric Light Company | Electric and Gas Division [Member] | Other Deferred Charges [Member]
Significant Accounting Policies [Line Items]
Hardship accounts in regulatory assets		$ 7.9	$ 6.8
Quarterly Payment [Member]
Significant Accounting Policies [Line Items]
Common stock dividend paid per share | $ / shares			$ 0.375	$ 0.37
Common stock dividend per share, declared | $ / shares		$ 0.38
Quarterly Payment [Member] | Subsequent Event [Member]
Significant Accounting Policies [Line Items]
Common stock dividend paid per share | $ / shares	0.39
Increase in dividend declared amount per share | $ / shares	$ 0.01
Annual Electric Sales Volume
Significant Accounting Policies [Line Items]
Percentage of total sales volumes revenue subject to RDM		27.00%
Annual Natural Gas Sales Volume
Significant Accounting Policies [Line Items]
Percentage of total sales volumes revenue subject to RDM		11.00%
Environmental and Rate Case Costs and Other Expenditures | Recovered over the next seven years
Significant Accounting Policies [Line Items]
Regulatory assets		$ 8.5
ISO-NE Obligations
Significant Accounting Policies [Line Items]
Cash Deposits		$ 2.7	$ 2.4
Maximum
Significant Accounting Policies [Line Items]
Lease term		12 months
Deferred Compensation Plan [Member]
Significant Accounting Policies [Line Items]
Investments in trading securities		$ 0.6	$ 0.5