Summary of Significant Accounting Policies - Additional Information (Detail) $ / shares in Units, $ in Millions	1 Months Ended	3 Months Ended								12 Months Ended
	Jan. 31, 2021 $ / shares	Dec. 31, 2020 USD ($) $ / shares	Sep. 30, 2020 $ / shares	Jun. 30, 2020 $ / shares	Mar. 31, 2020 $ / shares	Dec. 31, 2019 USD ($) $ / shares	Sep. 30, 2019 $ / shares	Jun. 30, 2019 $ / shares	Mar. 31, 2019 USD ($) $ / shares	Dec. 31, 2020 USD ($) mi Subsidiary Entity $ / shares	Dec. 31, 2019 USD ($) $ / shares	Dec. 31, 2018 USD ($) $ / shares	Dec. 31, 2017 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		$ 105.2				$ 96.0				$ 105.2	96.0
Regulatory assets		164.7				147.8				164.7	147.8
Investments in trading securities		5.7				5.6				$ 5.7	5.6
Number of Subsidiaries | Entity										3
Allowance for doubtful accounts		$ 3.3				1.0				$ 3.3	1.0	$ 1.3	$ 1.6
Power Supply Contract Divestitures						$ 0.3					$ 0.3
Depreciation rate based on average depreciable property balance										3.34%	3.41%	3.38%
Common stock dividend paid per share | $ / shares		$ 0.375	$ 0.375	$ 0.375	$ 0.375	$ 0.37	$ 0.37	$ 0.37	$ 0.37
Common stock dividend per share, declared | $ / shares										$ 1.50	$ 1.48	$ 1.46
Increase in dividend declared amount per share | $ / shares										$ 1.50
Inventory of materials and supplies		$ 0.6				$ 0.8				$ 0.6	$ 0.8
Average Interest Rate On Debt										3.12%	3.90%	2.70%
Materials And Supplies [Member]
Significant Accounting Policies [Line Items]
Inventory of materials and supplies		8.5				7.9				$ 8.5	$ 7.9
Subsequent Event [Member]
Significant Accounting Policies [Line Items]
Increase in dividend declared amount per share | $ / shares	$ 1.52
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				1.0				$ 3.1	$ 1.0
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
Power Supply Contract Divestitures		0.3								$ 0.3
Quarterly Payment [Member]
Significant Accounting Policies [Line Items]
Common stock dividend paid per share | $ / shares											$ 0.37	$ 0.365
Common stock dividend per share, declared | $ / shares										$ 0.375
Quarterly Payment [Member] | Subsequent Event [Member]
Significant Accounting Policies [Line Items]
Common stock dividend paid per share | $ / shares	0.38
Increase in dividend declared amount per share | $ / shares	$ 0.005
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		8.0								$ 8.0
ISO-NE Obligations
Significant Accounting Policies [Line Items]
Cash Deposits		$ 2.4				1.9				$ 2.4	$ 1.9
Maximum
Significant Accounting Policies [Line Items]
Lease term		12 months								12 months
Deferred Compensation Plan [Member]
Significant Accounting Policies [Line Items]
Investments in trading securities		$ 0.5				$ 0.2				$ 0.5	$ 0.2