XML 58 R44.htm IDEA: XBRL DOCUMENT
v3.3.1.900

Environmental Matters - Additional Information (Detail) | 3 Months Ended
--- | ---
Mar. 31, 2016 USD ($)
Maine | Environmental Restoration Costs |
Site Contingency [Line Items] |
Amortization period for environmental costs | 5 years
New Hampshire | Environmental Restoration Costs |
Site Contingency [Line Items] |
Amortization period for environmental costs | 7 years
Environmental Matters |
Site Contingency [Line Items] |
Income (loss) incurred in excess of recorded amount | $ 0
Fitchburg Gas and Electric Light Company | Environmental Restoration Costs |
Site Contingency [Line Items] |
Amortization period for environmental costs | 7 years

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_SiteContingencyLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Disclosure that no losses are anticipated exceeding those amounts recorded. + References No definition available. + Details
Name: | utl_ChangeInNetIncomeLossDueToChangeInEstimates
Namespace Prefix: | utl_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Remaining Amortization Period. + References No definition available. + Details
Name: | utl_RemainingAmortizationPeriod
Namespace Prefix: | utl_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_StatementGeographicalAxis=stpr_ME
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryLiabilityAxis=us-gaap_EnvironmentalRestorationCostsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementGeographicalAxis=stpr_NH
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_LossContingenciesByNatureOfContingencyAxis=utl_EnvironmentalMattersMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | dei_LegalEntityAxis=utl_FitchburgGasAndElectricLightCompanyMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |