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Energy Supply - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2014 MMBTU Bcf
Northern Utilities Inc
Gas and Oil Acreage [Line Items]
Natural gas available under firm contract per day of year-round and seasonal transportation and underground storage capacity to distribution facilities	100,000,000,000us-gaap_OilAndGasDeliveryCommitmentsAndContractsDailyProduction / us-gaap_BusinessAcquisitionAxis = utl_NorthernUtilitiesIncMember
Natural gas, underground storage	3,400,000,000utl_NaturalGasPipelineCapacityPerDay / us-gaap_BusinessAcquisitionAxis = utl_NorthernUtilitiesIncMember
Northern Utilities Inc | Maximum
Gas and Oil Acreage [Line Items]
Purchases of natural gas, contract duration	1 year
Fitchburg Gas and Electric Light Company
Gas and Oil Acreage [Line Items]
Natural gas available under firm contract per day of year-round and seasonal transportation and underground storage capacity to distribution facilities	14,057us-gaap_OilAndGasDeliveryCommitmentsAndContractsDailyProduction / us-gaap_BusinessAcquisitionAxis = utl_FitchburgGasAndElectricLightCompanyMember
Percentage of power supply requirement	50.00%utl_PowerSupplyCostBenefitVariancePercentage / us-gaap_BusinessAcquisitionAxis = utl_FitchburgGasAndElectricLightCompanyMember
Power supply contract duration	12 months
Fitchburg Gas and Electric Light Company | Maximum
Gas and Oil Acreage [Line Items]
Purchases of natural gas, contract duration	1 year
Unitil Energy Systems Inc
Gas and Oil Acreage [Line Items]
Percentage of power supply requirement	100.00%utl_PowerSupplyCostBenefitVariancePercentage / us-gaap_BusinessAcquisitionAxis = utl_UnitilEnergySystemsIncMember
Power supply contract duration	6 months

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- Definition

Amount of daily production of the applicable energy source.

+ References

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

-Publisher SEC

-Name Industry Guide

-Number 2

-Paragraph 8

-Subparagraph A

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Natural Gas Pipeline, Capacity Per Day

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Power Supply Agreement

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Power Supply Cost Benefit Variance Percentage

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