Summary of Significant Accounting Policies - Additional Information (Detail) Bcf in Billions	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) Bcf	Sep. 30, 2017 USD ($) Bcf	Sep. 30, 2018 USD ($) Subsidiary mi Bcf	Sep. 30, 2017 USD ($) Bcf	Dec. 31, 2017 USD ($) Bcf
Significant Accounting Policies [Line Items]
Length Of Pipeline | mi			86
Revenue sharing payments	$ 300,000	$ 300,000	$ 800,000	$ 800,000
Other	87,100,000	82,600,000	315,000,000	286,300,000
Operation and Maintenance Expense	16,400,000	16,900,000	51,500,000	49,400,000
Cost of removal obligation	91,200,000	84,800,000	91,200,000	84,800,000	$ 84,300,000
Regulatory Assets	$ 138,000,000	$ 133,300,000	$ 138,000,000	$ 133,300,000	$ 149,100,000
Number of Natural Gas Storage Outstanding | Bcf	0.0	1.2	0.0	1.2	0.6
Investments in trading securities	$ 5,300,000	$ 3,400,000	$ 5,300,000	$ 3,400,000	$ 3,600,000
ASU 2014-09
Significant Accounting Policies [Line Items]
Revenue sharing payments		300,000		800,000
Operation and Maintenance Expense		16,600,000		48,600,000
Non Service
Significant Accounting Policies [Line Items]
Non-service cost net periodic benefit costs	1,200,000	900,000	4,100,000	3,600,000
Not Designated as Hedging Instruments
Significant Accounting Policies [Line Items]
Derivative Assets, not designated as hedging instruments	$ 0	100,000	$ 0	100,000	100,000
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%
Maximum
Significant Accounting Policies [Line Items]
Cash equivalents maturity period	3 months
ISO-NE Obligations
Significant Accounting Policies [Line Items]
Cash Deposits	$ 3,200,000	4,800,000	$ 3,200,000	4,800,000	2,900,000
Natural Gas Hedging Program
Significant Accounting Policies [Line Items]
Cash Deposits	0	0	0	0	$ 0
Environmental and Rate Case Costs and Other Expenditures | Recovered over the next seven years
Significant Accounting Policies [Line Items]
Regulatory Assets	$ 6,100,000		$ 6,100,000
Restatement Adjustment [Member] | ASU 2014-09
Significant Accounting Policies [Line Items]
Other		(300,000)		(800,000)
Operation and Maintenance Expense		$ (300,000)		$ (800,000)
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