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Energy Supply Obligations (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Contractual Obligation [Line Items]
Energy Supply Obligations	$ 12.6us-gaap_EnergyMarketingContractLiabilitiesCurrent	$ 22.1us-gaap_EnergyMarketingContractLiabilitiesCurrent	$ 6.1us-gaap_EnergyMarketingContractLiabilitiesCurrent
Energy Supply Obligations	1.9us-gaap_EnergyMarketingContractLiabilitiesNoncurrent	1.9us-gaap_EnergyMarketingContractLiabilitiesNoncurrent	2.3us-gaap_EnergyMarketingContractLiabilitiesNoncurrent
Total Energy Supply Obligations	14.5utl_TotalEnergyMarketingContractLiabilities	24.0utl_TotalEnergyMarketingContractLiabilities	8.4utl_TotalEnergyMarketingContractLiabilities
Exchange Gas Obligation
Contractual Obligation [Line Items]
Energy Supply Obligations	3.6us-gaap_EnergyMarketingContractLiabilitiesCurrent / us-gaap_RegulatoryLiabilityAxis = us-gaap_ObligationsMember	14.2us-gaap_EnergyMarketingContractLiabilitiesCurrent / us-gaap_RegulatoryLiabilityAxis = us-gaap_ObligationsMember	1.0us-gaap_EnergyMarketingContractLiabilitiesCurrent / us-gaap_RegulatoryLiabilityAxis = us-gaap_ObligationsMember
Renewable Energy Portfolio Standards
Contractual Obligation [Line Items]
Energy Supply Obligations	8.5us-gaap_EnergyMarketingContractLiabilitiesCurrent / us-gaap_RegulatoryLiabilityAxis = utl_RenewableEnergyPortfolioRequirementMember	7.4us-gaap_EnergyMarketingContractLiabilitiesCurrent / us-gaap_RegulatoryLiabilityAxis = utl_RenewableEnergyPortfolioRequirementMember	4.3us-gaap_EnergyMarketingContractLiabilitiesCurrent / us-gaap_RegulatoryLiabilityAxis = utl_RenewableEnergyPortfolioRequirementMember
Power Supply Contract Divestitures
Contractual Obligation [Line Items]
Energy Supply Obligations	0.5us-gaap_EnergyMarketingContractLiabilitiesCurrent / us-gaap_RegulatoryLiabilityAxis = utl_DivestitureMember	0.5us-gaap_EnergyMarketingContractLiabilitiesCurrent / us-gaap_RegulatoryLiabilityAxis = utl_DivestitureMember	0.8us-gaap_EnergyMarketingContractLiabilitiesCurrent / us-gaap_RegulatoryLiabilityAxis = utl_DivestitureMember
Energy Supply Obligations	$ 1.9us-gaap_EnergyMarketingContractLiabilitiesNoncurrent / us-gaap_RegulatoryLiabilityAxis = utl_DivestitureMember	$ 1.9us-gaap_EnergyMarketingContractLiabilitiesNoncurrent / us-gaap_RegulatoryLiabilityAxis = utl_DivestitureMember	$ 2.3us-gaap_EnergyMarketingContractLiabilitiesNoncurrent / us-gaap_RegulatoryLiabilityAxis = utl_DivestitureMember

X

- Definition

Current liabilities attributable to energy marketing activities (trading activities). Current energy marketing (trading activities) are when entities engage in both financial and physical trading to increase profits, manage their commodity price risk and enhance system reliability. They may trade electricity, coal, natural gas and oil. These entities typically use a variety of financial instruments, including forward contracts, options and swaps.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.20)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

X

- Definition

Fair value of liability arising from energy marketing activities (trading activities), expected to be settled after one year or beyond the normal operating cycle, if longer. Examples of energy marketing activities include, but are not limited to, trading of electricity, coal, natural gas and oil using financial instruments, including, but not limited to, forward contracts, options and swaps.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 10

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13433-108611



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 45

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 15

-URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13495-108611



Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 45

-Paragraph 6

-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958

+ Details

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- Details

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- Definition

Total amount of obligations resulting from entry into energy supply contracts.

+ References

No definition available.

+ Details

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- Details

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- Details

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- Details

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