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Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2014 Bcf	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013 Bcf	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014 Entity mi Bcf	Dec. 31, 2013 Bcf	Dec. 31, 2012	Dec. 31, 2015
Significant Accounting Policies [Line Items]
Number of Subsidiaries									3utl_NumberOfSubsidiaries
Length Of Pipeline									86utl_LengthOfPipeline
Depreciation rate based on average depreciable property balance									3.56%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService	3.59%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService	3.60%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService
Common stock dividend paid per share	$ 0.345us-gaap_CommonStockDividendsPerShareCashPaid	$ 0.345us-gaap_CommonStockDividendsPerShareCashPaid	$ 0.345us-gaap_CommonStockDividendsPerShareCashPaid	$ 0.345us-gaap_CommonStockDividendsPerShareCashPaid	$ 0.345us-gaap_CommonStockDividendsPerShareCashPaid	$ 0.345us-gaap_CommonStockDividendsPerShareCashPaid	$ 0.345us-gaap_CommonStockDividendsPerShareCashPaid	$ 0.345us-gaap_CommonStockDividendsPerShareCashPaid	$ 1.38us-gaap_CommonStockDividendsPerShareCashPaid	$ 1.38us-gaap_CommonStockDividendsPerShareCashPaid	$ 1.38us-gaap_CommonStockDividendsPerShareCashPaid
Average interest rates									1.56%utl_AverageInterestRateOnDebt	1.92%utl_AverageInterestRateOnDebt	2.04%utl_AverageInterestRateOnDebt
Cost of removal obligation	$ 63,800,000us-gaap_AssetRetirementObligationsNoncurrent				$ 57,300,000us-gaap_AssetRetirementObligationsNoncurrent				$ 63,800,000us-gaap_AssetRetirementObligationsNoncurrent	$ 57,300,000us-gaap_AssetRetirementObligationsNoncurrent
Number of Natural Gas Storage Outstanding	2.4utl_NaturalGasAndOilProvedReservesMultiplierToCoverFairValueOfTransactionsOutstanding				1.8utl_NaturalGasAndOilProvedReservesMultiplierToCoverFairValueOfTransactionsOutstanding				2.4utl_NaturalGasAndOilProvedReservesMultiplierToCoverFairValueOfTransactionsOutstanding	1.8utl_NaturalGasAndOilProvedReservesMultiplierToCoverFairValueOfTransactionsOutstanding
Intangible Assets, Purchase Adjustments									9,700,000us-gaap_IndefiniteLivedIntangibleAssetsTranslationAndPurchaseAccountingAdjustments
Plant
Significant Accounting Policies [Line Items]
Intangible Asset Amortization Period									10 years
Bargain Purchase Adjustment
Significant Accounting Policies [Line Items]
Intangible Asset Amortization Period									4 years
Maximum
Significant Accounting Policies [Line Items]
Cash equivalents maturity period									3 months
ISO-NE Obligations
Significant Accounting Policies [Line Items]
Cash Deposits	6,300,000utl_CashDeposits / us-gaap_CashAndCashEquivalentsAxis = utl_NewEnglandMember				7,300,000utl_CashDeposits / us-gaap_CashAndCashEquivalentsAxis = utl_NewEnglandMember				6,300,000utl_CashDeposits / us-gaap_CashAndCashEquivalentsAxis = utl_NewEnglandMember	7,300,000utl_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				$ 0utl_CashDeposits / us-gaap_CashAndCashEquivalentsAxis = utl_HedgingObligationsMember	$ 0utl_CashDeposits / us-gaap_CashAndCashEquivalentsAxis = utl_HedgingObligationsMember
Annual Electric Sales Volume
Significant Accounting Policies [Line Items]
Percentage of total sales volumes revenue subject to RDM												27.00%utl_PercentageOfSalesVolume / us-gaap_ProductOrServiceAxis = utl_ProductTypeOneMember
Annual Natural Gas Sales Volume
Significant Accounting Policies [Line Items]
Percentage of total sales volumes revenue subject to RDM												11.00%utl_PercentageOfSalesVolume / us-gaap_ProductOrServiceAxis = utl_ProductTypeTwoMember
Quarterly Dividends
Significant Accounting Policies [Line Items]
Common stock dividend paid per share									$ 0.345us-gaap_CommonStockDividendsPerShareCashPaid / utl_PeriodAxis = utl_QuarterlyPaymentMember	$ 0.345us-gaap_CommonStockDividendsPerShareCashPaid / utl_PeriodAxis = utl_QuarterlyPaymentMember	$ 0.345us-gaap_CommonStockDividendsPerShareCashPaid / utl_PeriodAxis = utl_QuarterlyPaymentMember
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

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

X

- Definition

Aggregate dividends paid during the period for each share of common stock outstanding.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Article 3



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.3-04)

-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

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Namespace Prefix:	us-gaap_
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Balance Type:	na
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X

- Definition

Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	na
Period Type:	duration

X

- Definition

Amount of increase (decrease) of assets, excluding financial assets and goodwill, lacking physical substance and having a projected indefinite period of benefit, from foreign currency translation and purchase accounting adjustments.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	debit
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X

- Definition

Composite depreciation rate for public utility plants in service.

+ References

No definition available.

+ Details

Name:	us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Average Interest Rate On Debt

+ References

No definition available.

+ Details

Name:	utl_AverageInterestRateOnDebt
Namespace Prefix:	utl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Cash Deposits

+ References

No definition available.

+ Details

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

X

- Definition

Cash Equivalents, Maturity Period

+ References

No definition available.

+ Details

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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

- Definition

Percentage Of Sales Volume

+ References

No definition available.

+ Details

Name:	utl_PercentageOfSalesVolume
Namespace Prefix:	utl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	utl_SignificantAccountingPoliciesLineItems
Namespace Prefix:	utl_
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- Details

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Namespace Prefix:
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Period Type:

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- Details

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Namespace Prefix:
Data Type:	na
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Period Type:

X

- Details

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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_MajorPropertyClassAxis=utl_BargainPurchaseAdjustmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_MajorPropertyClassAxis=utl_PlantMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_ProductOrServiceAxis=utl_ProductTypeOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

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- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

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- Details

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

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- Details

Name:	utl_PeriodAxis=utl_QuarterlyPaymentMember
Namespace Prefix:
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Balance Type:
Period Type: