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Regulatory Matters - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	1 Months Ended				3 Months Ended		1 Months Ended
	Oct. 31, 2012 Fitchburg Gas and Electric Light Company	May 01, 2012 Fitchburg Gas and Electric Light Company	Feb. 28, 2013 Unitil Energy Systems Inc	Oct. 31, 2012 Unitil Energy Systems Inc	Mar. 31, 2013 Unitil Energy Systems Inc	Mar. 14, 2013 Unitil Energy Systems Inc Storm Costs	Apr. 26, 2011 Unitil Energy Systems Inc Year One Policy	Apr. 26, 2011 Unitil Energy Systems Inc Year Two Policy	Apr. 30, 2013 Northern Utilities Inc Subsequent Event LegalMatter	Apr. 30, 2013 Northern Utilities Inc New Hampshire Operation Subsequent Event	Apr. 30, 2013 Northern Utilities Inc Maine Operations Subsequent Event
Regulatory Assets [Line Items]
Increase in annual revenue							$ 5.2	$ 5.0	$ 2.5	$ 5.2	$ 4.6
Amendment effective date							July 1, 2010	May 1, 2011	July 1, 2013
Settlement agreement date					May 01, 2016
Adjustment to recover the increased spending for vegetation management program and reliability enhancement program and a proposed storm resiliency program			2.8
Number of cases filed									2
Percent of increase in annual revenue										9.40%	6.30%
Recovery of damage of electrical system	1.1			2.7
Construction Expenditures	0.3			0.4
Storm expenditures		4.3
Recovery of damage of electrical system						$ 2.3

X

- Definition

Carrying amount as of the balance sheet date of the portion of the reserved funds established by a utility from customer payments to cover the repair costs of damages from major storms, which repairs are expected to be incurred beyond one year or the normal operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 71

-Paragraph 9

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 980

-SubTopic 340

-Section 25

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6499739&loc=d3e43603-110378

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X

- Definition

Future cash outflow to pay for construction in progress expenditures that have occurred.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 50

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4313-108586



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 50

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4304-108586



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 50

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4332-108586

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X

- Definition

The increase (decrease) during the reporting period in unrecovered fuel costs for a regulated utility company.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 95

-Paragraph 28

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585

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- Details

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X

- Definition

A noncash adjustment to the provision established for storm damage by an oil, gas, or utility company.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 95

-Paragraph 28

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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Balance Type:	debit
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X

- Definition

Adjustment for rate Increase

+ References

No definition available.

+ Details

Name:	utl_AdjustmentForRateIncrease
Namespace Prefix:	utl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Amendment Effective Date

+ References

No definition available.

+ Details

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Data Type:	us-types:dateStringItemType
Balance Type:	na
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X

- Definition

Number Of Cases Filed

+ References

No definition available.

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- Definition

Percentage Of Revenue Increase

+ References

No definition available.

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- Definition

Settlement Dates

+ References

No definition available.

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X

- Definition

Expenditures related to specific storms.

+ References

No definition available.

+ Details

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