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Fair Value Amount of Derivative Assets Liabilities Offset in Regulatory Liabilities Assets (Detail) (Not Designated as Hedging Instrument, USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Derivative Assets |  |
Derivative Assets | $ 0.1 us-gaap_DerivativeFairValueOfDerivativeAssetAmountNotOffsetAgainstCollateral | $ 0.2 us-gaap_DerivativeFairValueOfDerivativeAssetAmountNotOffsetAgainstCollateral
Derivative Liabilities |  |
Derivative Liabilities | 0 us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral | 0 us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral
Natural Gas Future/Options Contract | Prepayments and Other |  |
Derivative Assets |  |
Derivative Assets |  | 0.1 us-gaap_DerivativeFairValueOfDerivativeAssetAmountNotOffsetAgainstCollateral / us-gaap_BalanceSheetLocationAxis = utl_PrepaymentAndOtherCurrentAssetsMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember / dei_LegalEntityAxis = utl_NaturalGasPurchaseContractMember
Natural Gas Future/Options Contract | Other Noncurrent Assets |  |
Derivative Assets |  |
Derivative Assets | 0.1 us-gaap_DerivativeFairValueOfDerivativeAssetAmountNotOffsetAgainstCollateral / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherNoncurrentAssetsMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember / dei_LegalEntityAxis = utl_NaturalGasPurchaseContractMember | 0.1 us-gaap_DerivativeFairValueOfDerivativeAssetAmountNotOffsetAgainstCollateral / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherNoncurrentAssetsMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember / dei_LegalEntityAxis = utl_NaturalGasPurchaseContractMember
Natural Gas Future/Options Contract | Other Current Liabilities |  |
Derivative Liabilities |  |
Derivative Liabilities | 0 us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherCurrentLiabilitiesMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember / dei_LegalEntityAxis = utl_NaturalGasPurchaseContractMember | 0 us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherCurrentLiabilitiesMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember / dei_LegalEntityAxis = utl_NaturalGasPurchaseContractMember
Natural Gas Future/Options Contract | Other Noncurrent Liabilities |  |
Derivative Liabilities |  |
Derivative Liabilities | $ 0 us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherNoncurrentLiabilitiesMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember / dei_LegalEntityAxis = utl_NaturalGasPurchaseContractMember | $ 0 us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherNoncurrentLiabilitiesMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember / dei_LegalEntityAxis = utl_NaturalGasPurchaseContractMember

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- Definition Fair value, after offset of derivative liability, of financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, before offset against an obligation to return collateral under a master netting arrangement. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=34720828&loc=SL20225862-175312 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=36459792&loc=SL20226008-175313 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 9 -URI http://asc.fasb.org/extlink&oid=36459792&loc=SL20226006-175313 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=35706170&loc=d3e41678-113959 + Details
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- Definition Fair value, after offset of derivative asset, of financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, elected not to be and before offset against a right to receive collateral under a master netting arrangement. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=34720828&loc=SL20225862-175312 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=36459792&loc=SL20226008-175313 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 9 -URI http://asc.fasb.org/extlink&oid=36459792&loc=SL20226006-175313 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=35706170&loc=d3e41678-113959 + Details
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