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Summary of Significant Accounting Policies - Additional Information (Detail) Bcf in Billions	9 Months Ended
	Sep. 30, 2017 USD ($) Subsidiary mi Bcf	Dec. 31, 2016 USD ($) Bcf	Sep. 30, 2016 USD ($) Bcf
Significant Accounting Policies [Line Items]
Length Of Pipeline | mi	86
Cost of removal obligation	$ 84,800,000	$ 77,000,000	$ 77,400,000
Regulatory Assets	$ 133,300,000	$ 142,000,000	$ 124,500,000
Number of Natural Gas Storage Outstanding | Bcf	1.2	2.0	2.9
Investments in trading securities	$ 3,400,000	$ 1,900,000	$ 1,900,000
Not Designated as Hedging Instruments
Significant Accounting Policies [Line Items]
Derivative Assets, not designated as hedging instruments	$ 100,000	400,000	200,000
Maximum
Significant Accounting Policies [Line Items]
Cash equivalents maturity period	3 months
ISO-NE Obligations
Significant Accounting Policies [Line Items]
Cash Deposits	$ 4,800,000	2,800,000	3,500,000
Natural Gas Hedging Program
Significant Accounting Policies [Line Items]
Cash Deposits	0	0	0
Deferred Storm Charges
Significant Accounting Policies [Line Items]
Regulatory Assets	6,800,000	$ 9,600,000	$ 11,000,000
Deferred Storm Charges | Recovered over Next Year
Significant Accounting Policies [Line Items]
Regulatory Assets	1,400,000
Environmental and Rate Case Costs and Other Expenditures | Recovered over the next seven years
Significant Accounting Policies [Line Items]
Regulatory Assets	$ 7,400,000
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

X

- Definition

Noncurrent portion of the carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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- Definition

The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 340
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-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382

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Amount of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.

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Cash Deposits

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Cash Equivalents, Maturity Period

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Length of natural gas transmission pipeline.

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- Definition

Amount of natural gas purchase contracts outstanding under hedging program.

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