Summary of Significant Accounting Policies - Additional Information (Detail) Bcf in Billions	3 Months Ended
	Mar. 31, 2018 USD ($) Subsidiary mi Bcf	Mar. 31, 2017 USD ($) Bcf	Dec. 31, 2017 USD ($) Bcf
Significant Accounting Policies [Line Items]
Length Of Pipeline | mi	86
Revenue sharing payments	$ 300,000	$ 300,000
Other	1,300,000	1,700,000
Operation and Maintenance Expense	17,300,000	16,000,000
Cost of removal obligation	86,600,000	79,700,000	$ 84,300,000
Regulatory Assets	$ 145,800,000	$ 135,500,000	$ 149,100,000
Number of Natural Gas Storage Outstanding | Bcf	0.0	1.2	0.6
Investments in trading securities	$ 5,100,000	$ 3,300,000	$ 3,600,000
ASU 2014-09
Significant Accounting Policies [Line Items]
Revenue sharing payments		300,000
Operation and Maintenance Expense		15,700,000
Non Service
Significant Accounting Policies [Line Items]
Non-service cost net periodic benefit costs	1,500,000	1,600,000
Not Designated as Hedging Instruments
Significant Accounting Policies [Line Items]
Derivative Assets, not designated as hedging instruments	$ 0	300,000	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	$ 3,300,000	2,600,000	2,900,000
Natural Gas Hedging Program
Significant Accounting Policies [Line Items]
Cash Deposits	0	0	0
Restatement Adjustment [Member] | ASU 2014-09
Significant Accounting Policies [Line Items]
Other		(300,000)
Operation and Maintenance Expense		(300,000)
Deferred Storm Charges
Significant Accounting Policies [Line Items]
Regulatory Assets	8,000,000	$ 8,600,000	$ 7,200,000
Deferred Storm Charges | Recovered over Next Year
Significant Accounting Policies [Line Items]
Regulatory Assets	500,000
Environmental and Rate Case Costs and Other Expenditures | Recovered over the next seven years
Significant Accounting Policies [Line Items]
Regulatory Assets	$ 6,500,000
Unitil Service; Unitil Realty; and Unitil Resources
Significant Accounting Policies [Line Items]
Number of Subsidiaries | Subsidiary	3
Utilities
Significant Accounting Policies [Line Items]
Number of Subsidiaries | Subsidiary	3