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Summary of Significant Accounting Policies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015 mi Bcf	Dec. 31, 2014 Bcf	Mar. 31, 2014 Bcf
Significant Accounting Policies [Line Items]
Length Of Pipeline	86utl_LengthOfPipeline
Cost of Removal Obligations	$ 65.7us-gaap_AssetRetirementObligationsNoncurrent	$ 63.8us-gaap_AssetRetirementObligationsNoncurrent	$ 59.2us-gaap_AssetRetirementObligationsNoncurrent
Number of Natural Gas Storage Outstanding	2.0utl_NaturalGasAndOilProvedReservesMultiplierToCoverFairValueOfTransactionsOutstanding	2.4utl_NaturalGasAndOilProvedReservesMultiplierToCoverFairValueOfTransactionsOutstanding	1.1utl_NaturalGasAndOilProvedReservesMultiplierToCoverFairValueOfTransactionsOutstanding
Maximum
Significant Accounting Policies [Line Items]
Cash equivalents maturity period	3 months
ISO-NE Obligations
Significant Accounting Policies [Line Items]
Cash Deposits	1.5utl_CashDeposits / us-gaap_CashAndCashEquivalentsAxis = utl_NewEnglandMember	6.3utl_CashDeposits / us-gaap_CashAndCashEquivalentsAxis = utl_NewEnglandMember	9.8utl_CashDeposits / us-gaap_CashAndCashEquivalentsAxis = utl_NewEnglandMember
Natural Gas Hedging Program
Significant Accounting Policies [Line Items]
Cash Deposits	$ 0utl_CashDeposits / us-gaap_CashAndCashEquivalentsAxis = utl_HedgingObligationsMember	$ 0utl_CashDeposits / us-gaap_CashAndCashEquivalentsAxis = utl_HedgingObligationsMember	$ 0.1utl_CashDeposits / us-gaap_CashAndCashEquivalentsAxis = utl_HedgingObligationsMember
Utilities
Significant Accounting Policies [Line Items]
Number of Subsidiaries	3utl_NumberOfSubsidiaries / dei_LegalEntityAxis = utl_UtilitiesMember
Other Subsidiaries
Significant Accounting Policies [Line Items]
Number of Subsidiaries	3utl_NumberOfSubsidiaries / dei_LegalEntityAxis = utl_OtherSubsidiariesMember

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

+ Details

Name:	us-gaap_AssetRetirementObligationsNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Cash Deposits

+ References

No definition available.

+ Details

Name:	utl_CashDeposits
Namespace Prefix:	utl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Cash Equivalents, Maturity Period

+ References

No definition available.

+ Details

Name:	utl_CashEquivalentsMaturityPeriod
Namespace Prefix:	utl_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Length of natural gas transmission pipeline.

+ References

No definition available.

+ Details

Name:	utl_LengthOfPipeline
Namespace Prefix:	utl_
Data Type:	num:lengthItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Amount of natural gas purchase contracts outstanding under hedging program.

+ References

No definition available.

+ Details

Name:	utl_NaturalGasAndOilProvedReservesMultiplierToCoverFairValueOfTransactionsOutstanding
Namespace Prefix:	utl_
Data Type:	num:volumeItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Number of Subsidiaries.

+ References

No definition available.

+ Details

Name:	utl_NumberOfSubsidiaries
Namespace Prefix:	utl_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	utl_SignificantAccountingPoliciesLineItems
Namespace Prefix:	utl_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	dei_LegalEntityAxis=utl_OtherSubsidiariesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	dei_LegalEntityAxis=utl_UtilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_CashAndCashEquivalentsAxis=utl_HedgingObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_CashAndCashEquivalentsAxis=utl_NewEnglandMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: