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Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2015 USD ($) Subsidiary mi Bcf	Dec. 31, 2014 USD ($) Bcf	Sep. 30, 2014 USD ($) Bcf
Significant Accounting Policies [Line Items]
Length Of Pipeline | mi	86
Cost of Removal Obligations	$ 69.9	$ 63.8	$ 63.2
Number of Natural Gas Storage Outstanding | Bcf	3.7	2.4	3.0
Maximum
Significant Accounting Policies [Line Items]
Cash equivalents maturity period	3 months
ISO-NE Obligations
Significant Accounting Policies [Line Items]
Cash Deposits	$ 2.9	$ 6.3	$ 8.0
Natural Gas Hedging Program
Significant Accounting Policies [Line Items]
Cash Deposits	$ 0.0	$ 0.0	$ 0.0
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

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)



-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

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Balance Type:	credit
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- Definition

Cash Deposits

+ References

No definition available.

+ Details

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Balance Type:	debit
Period Type:	instant

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- Definition

Cash Equivalents, Maturity Period

+ References

No definition available.

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- Definition

Length of natural gas transmission pipeline.

+ References

No definition available.

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- Definition

Amount of natural gas purchase contracts outstanding under hedging program.

+ References

No definition available.

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- Definition

Number of Subsidiaries.

+ References

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- Definition

Significant Accounting Policies [Line Items]

+ References

No definition available.

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- Details

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