Summary of Significant Accounting Policies - Additional Information (Detail) $ / shares in Units, $ in Millions	1 Months Ended	3 Months Ended								12 Months Ended
	Jan. 31, 2020 $ / shares	Dec. 31, 2019 USD ($) $ / shares	Sep. 30, 2019 $ / shares	Jun. 30, 2019 $ / shares	Mar. 31, 2019 $ / shares	Dec. 31, 2018 USD ($) $ / shares	Sep. 30, 2018 $ / shares	Jun. 30, 2018 $ / shares	Mar. 31, 2018 $ / shares	Dec. 31, 2019 USD ($) Entity Subsidiary mi $ / shares	Dec. 31, 2018 USD ($) $ / shares	Dec. 31, 2017 USD ($) $ / shares	Jan. 01, 2019 USD ($)
Significant Accounting Policies [Line Items]
Number of Subsidiaries | Entity										3
Length Of Pipeline | mi										86
Revenue sharing payments										$ 0.2	$ 1.0	$ 1.1
Depreciation rate based on average depreciable property balance										3.41%	3.38%	3.45%
Common stock dividend paid per share | $ / shares		$ 0.37	$ 0.37	$ 0.37	$ 0.37	$ 0.365	$ 0.365	$ 0.365	$ 0.365	$ 1.48	$ 1.46	$ 1.44
Common stock dividend per share, declared | $ / shares										$ 1.48	$ 1.46	$ 1.44
Inventory of materials and supplies		$ 0.8				$ 0.8				$ 0.8	$ 0.8
Average interest rates										3.90%	2.70%	2.90%
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		96.0				90.7				96.0	$ 90.7
Recognition of lease assets													$ 4.2
Recognition of lease liabilities													$ 4.2
Power Supply Contract Divestitures		0.3				0.6				0.3	0.6
Investments in trading securities		5.6				4.8				$ 5.6	$ 4.8
Quarterly Dividends
Significant Accounting Policies [Line Items]
Common stock dividend paid per share | $ / shares										$ 0.37	$ 0.365	$ 0.36
Materials And Supplies
Significant Accounting Policies [Line Items]
Inventory of materials and supplies		7.9				7.0				$ 7.9	$ 7.0
ASU 2014-09
Significant Accounting Policies [Line Items]
Revenue sharing payments												$ 1.1
Non Service
Significant Accounting Policies [Line Items]
Non-service cost net periodic benefit costs										$ 4.6	5.5	$ 5.7
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		0.6								$ 0.6
Subsequent Event
Significant Accounting Policies [Line Items]
Common stock dividend per share, declared | $ / shares	$ 1.50
Subsequent Event | Quarterly Dividends
Significant Accounting Policies [Line Items]
Common stock dividend per share, declared | $ / shares	0.375
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]
Power Supply Contract Divestitures		7.6								$ 7.6
ISO-NE Obligations
Significant Accounting Policies [Line Items]
Cash Deposits		$ 1.9				3.5				$ 1.9	3.5
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		$ 0.2				$ 0.0				$ 0.2	$ 0.0