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Summary of Significant Accounting Policies - Additional Information (Detail) Bcf in Billions		3 Months Ended
	Apr. 21, 2017 USD ($)	Mar. 31, 2017 USD ($) Subsidiary mi Bcf	Dec. 31, 2016 USD ($) Bcf	Mar. 31, 2016 USD ($) Bcf
Significant Accounting Policies [Line Items]
Length Of Pipeline \| mi		86
Cost of removal obligation		$ 79,700,000	$ 77,000,000	$ 72,300,000
Regulatory Assets		$ 135,500,000	$ 142,000,000	$ 131,900,000
Number of Natural Gas Storage Outstanding \| Bcf		1.2	2.0	1.8
Investments in trading securities		$ 3,300,000	$ 1,900,000	$ 700,000
Not Designated as Hedging Instruments
Significant Accounting Policies [Line Items]
Derivative Assets, not designated as hedging instruments		$ 300,000		100,000
Maximum
Significant Accounting Policies [Line Items]
Cash equivalents maturity period		3 months
ISO-NE Obligations
Significant Accounting Policies [Line Items]
Cash Deposits		$ 2,600,000	2,800,000	2,100,000
Natural Gas Hedging Program
Significant Accounting Policies [Line Items]
Cash Deposits		0	0	0
Subsequent Event \| Private Placement \| Senior Unsecured Notes
Significant Accounting Policies [Line Items]
Senior Notes	$ 90,000,000
Deferred Storm Charges
Significant Accounting Policies [Line Items]
Regulatory Assets		8,600,000	$ 9,600,000	$ 13,800,000
Deferred Storm Charges \| Recovered over Next One and a Half Years
Significant Accounting Policies [Line Items]
Regulatory Assets		2,500,000
Environmental and Rate Case Costs and Other Expenditures \| Recovered over the next seven and a half years
Significant Accounting Policies [Line Items]
Regulatory Assets		$ 8,000,000
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
Northern Utilities Inc \| Subsequent Event \| Senior Unsecured Notes 3.52% Due 2027
Significant Accounting Policies [Line Items]
Senior Notes	$ 20,000,000
Debt instrument due date	2027
Stated interest rate	3.52%
Northern Utilities Inc \| Subsequent Event \| Senior Unsecured Notes 4.32% Due 2047
Significant Accounting Policies [Line Items]
Senior Notes	$ 30,000,000
Debt instrument due date	2047
Stated interest rate	4.32%
Fitchburg Gas and Electric Light Company \| Subsequent Event \| Senior Unsecured Notes 3.52% Due 2027
Significant Accounting Policies [Line Items]
Senior Notes	$ 10,000,000
Debt instrument due date	2027
Stated interest rate	3.52%
Fitchburg Gas and Electric Light Company \| Subsequent Event \| Senior Unsecured Notes 4.32% Due 2047
Significant Accounting Policies [Line Items]
Senior Notes	$ 15,000,000
Debt instrument due date	2047
Stated interest rate	4.32%
Granite State Gas Transmission Inc \| Subsequent Event \| Senior Unsecured Notes 3.52% Due 2027
Significant Accounting Policies [Line Items]
Senior Notes	$ 15,000,000
Debt instrument due date	2027
Stated interest rate	3.72%