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Environmental Obligations Recognized by Company (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Site Contingency [Line Items]
Beginning Balance	$ 5.5utl_AccruedEnvironmentalLossContingenciesCurrentBeginningBalance	$ 14.8utl_AccruedEnvironmentalLossContingenciesCurrentBeginningBalance
Additions	0.1us-gaap_AccrualForEnvironmentalLossContingenciesProvisionForNewLosses	1.0us-gaap_AccrualForEnvironmentalLossContingenciesProvisionForNewLosses
Less: Payments / Reductions	0.2us-gaap_AccrualForEnvironmentalLossContingenciesPayments	1.6us-gaap_AccrualForEnvironmentalLossContingenciesPayments
Less: Current Portion	3.4us-gaap_AccruedEnvironmentalLossContingenciesCurrent	1.4us-gaap_AccruedEnvironmentalLossContingenciesCurrent	3.5us-gaap_AccruedEnvironmentalLossContingenciesCurrent
Ending Balance	2.0utl_AccruedEnvironmentalLossContingenciesNoncurrentEndingBalance	12.8utl_AccruedEnvironmentalLossContingenciesNoncurrentEndingBalance
Adjustment
Site Contingency [Line Items]
Ending Balance	5.4utl_AccruedEnvironmentalLossContingenciesNoncurrentEndingBalance / us-gaap_StatementScenarioAxis = us-gaap_ScenarioAdjustmentMember	14.2utl_AccruedEnvironmentalLossContingenciesNoncurrentEndingBalance / us-gaap_StatementScenarioAxis = us-gaap_ScenarioAdjustmentMember
Fitchburg Gas and Electric Light Company
Site Contingency [Line Items]
Beginning Balance	1.9utl_AccruedEnvironmentalLossContingenciesCurrentBeginningBalance / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember	12.0utl_AccruedEnvironmentalLossContingenciesCurrentBeginningBalance / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember
Additions		1.0us-gaap_AccrualForEnvironmentalLossContingenciesProvisionForNewLosses / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember
Less: Payments / Reductions	0.1us-gaap_AccrualForEnvironmentalLossContingenciesPayments / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember	1.0us-gaap_AccrualForEnvironmentalLossContingenciesPayments / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember
Less: Current Portion	1.8us-gaap_AccruedEnvironmentalLossContingenciesCurrent / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember	1.0us-gaap_AccruedEnvironmentalLossContingenciesCurrent / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember
Ending Balance		11.0utl_AccruedEnvironmentalLossContingenciesNoncurrentEndingBalance / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember
Fitchburg Gas and Electric Light Company | Adjustment
Site Contingency [Line Items]
Ending Balance	1.8utl_AccruedEnvironmentalLossContingenciesNoncurrentEndingBalance / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioAdjustmentMember	12.0utl_AccruedEnvironmentalLossContingenciesNoncurrentEndingBalance / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioAdjustmentMember
Northern Utilities Inc
Site Contingency [Line Items]
Beginning Balance	3.6utl_AccruedEnvironmentalLossContingenciesCurrentBeginningBalance / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember	2.8utl_AccruedEnvironmentalLossContingenciesCurrentBeginningBalance / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember
Additions	0.1us-gaap_AccrualForEnvironmentalLossContingenciesProvisionForNewLosses / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember
Less: Payments / Reductions	0.1us-gaap_AccrualForEnvironmentalLossContingenciesPayments / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember	0.6us-gaap_AccrualForEnvironmentalLossContingenciesPayments / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember
Less: Current Portion	1.6us-gaap_AccruedEnvironmentalLossContingenciesCurrent / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember	0.4us-gaap_AccruedEnvironmentalLossContingenciesCurrent / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember
Ending Balance	2.0utl_AccruedEnvironmentalLossContingenciesNoncurrentEndingBalance / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember	1.8utl_AccruedEnvironmentalLossContingenciesNoncurrentEndingBalance / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember
Northern Utilities Inc | Adjustment
Site Contingency [Line Items]
Ending Balance	$ 3.6utl_AccruedEnvironmentalLossContingenciesNoncurrentEndingBalance / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioAdjustmentMember	$ 2.2utl_AccruedEnvironmentalLossContingenciesNoncurrentEndingBalance / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioAdjustmentMember

X

- Definition

The aggregate amount disbursed during the reporting period to settle environmental loss contingencies.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 450

-SubTopic 20

-Section S99

-Paragraph 1

-Subparagraph (SAB TOPIC 5.Y.Q2)

-URI http://asc.fasb.org/extlink&oid=27011672&loc=d3e149879-122751

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X

- Definition

The amount added to the accrual for newly incurred environmental loss contingencies. These additions include remediation expense and interest accretion. These additions do not relate to changes in the environmental accrual related to a business acquisition or divestiture, to currency movements, or revisions in previously existing estimates.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 450

-SubTopic 20

-Section S99

-Paragraph 1

-Subparagraph (SAB TOPIC 5.Y.Q2)

-URI http://asc.fasb.org/extlink&oid=27011672&loc=d3e149879-122751

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X

- Definition

Carrying value of the obligation (known or estimated) arising from requirements to perform activities to remediate one or more sites, payable in twelve months or in the next operating cycle if longer.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 20

-Article 5



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 410

-SubTopic 30

-Section 50

-Paragraph 7

-URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13207-110859



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 450

-SubTopic 20

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14326-108349



Reference 4: 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



Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section 45

-Paragraph 6

-URI http://asc.fasb.org/extlink&oid=28358313&loc=d3e6911-107765



Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 410

-SubTopic 30

-Section 50

-Paragraph 10

-Subparagraph (d)(1)

-URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13237-110859

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- Definition

Accrued Environmental Loss Contingencies Current Beginning Balance

+ References

No definition available.

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- Definition

Accrued Environmental Loss Contingencies, Noncurrent Ending Balance

+ References

No definition available.

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