XML 65 R25.htm IDEA: XBRL DOCUMENT
v3.19.3

Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions | 3 Months Ended | 9 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Sep. 30, 2019 USD ($) mi | Sep. 30, 2019 USD ($) Subsidiary | Jan. 01, 2019 USD ($) | Dec. 31, 2018 USD ($) | Sep. 30, 2018 USD ($)
Significant Accounting Policies [Line Items] |  |  |  |  |
Length Of Pipeline | mi | 86 |  |  |  |
Gain on divestiture of business, pretax |  | $ 13.4 |  |  |
Gain on divestiture of business, net |  | 9.8 |  |  |
Provision for income taxes |  | 3.6 |  |  |
Cost of removal obligation | $ 98.4 | 98.4 |  | $ 90.7 | $ 91.2
Recognition of lease assets |  |  | $ 4.2 |  |
Recognition of lease liabilities |  |  | $ 4.2 |  |
Regulatory assets | 123.6 | 123.6 |  | 140.3 | 138.0
Investments in trading securities | 5.9 | $ 5.9 |  | 4.8 | 5.3
Utilities |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Number of Subsidiaries | Subsidiary |  | 3 |  |  |
Unitil Service; Unitil Realty; and Unitil Resources |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Number of Subsidiaries | Subsidiary |  | 3 |  |  |
Annual Electric Sales Volume |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Percentage of total sales volumes revenue subject to RDM |  | 27.00% |  |  |
Annual Natural Gas Sales Volume |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Percentage of total sales volumes revenue subject to RDM |  | 11.00% |  |  |
Environmental and Rate Case Costs and Other Expenditures | Recovered over the next seven years |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Regulatory assets | 5.8 | $ 5.8 |  |  |
ISO-NE Obligations |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Cash Deposits | $ 3.5 | $ 3.5 |  | 3.5 | 3.2
Maximum |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Cash equivalents maturity period |  | 3 months |  |  |
Lease term | 12 months | 12 months |  |  |
Deferred Compensation Plan [Member] |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Investments in trading securities | $ 0.1 | $ 0.1 |  | $ 0.0 | $ 0.0

X
- Definition Noncurrent portion of the carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + Details
Name: | us-gaap_AssetRetirementObligationsNoncurrent
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Term of lessee's operating lease, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 842 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=77888426&loc=SL77918673-209980 + Details
Name: | us-gaap_LesseeOperatingLeaseTermOfContract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | instant

X
- Definition The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 + Details
Name: | us-gaap_RegulatoryAssets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading) and investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.4) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
Name: | us-gaap_TradingSecurities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Cash Deposits + References No definition available. + Details
Name: | utl_CashDeposits
Namespace Prefix: | utl_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Cash Equivalents, Maturity Period + References No definition available. + Details
Name: | utl_CashEquivalentsMaturityPeriod
Namespace Prefix: | utl_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Disposal Group Including Discontinued Operations Tax Effect Of Gain On Divestiture + References No definition available. + Details
Name: | utl_DisposalGroupIncludingDiscontinuedOperationsTaxEffectOfGainOnDivestiture
Namespace Prefix: | utl_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Gain on divestiture of business pretax. + References No definition available. + Details
Name: | utl_GainOnDivestitureOfBusinessPretax
Namespace Prefix: | utl_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Lease assets. + References No definition available. + Details
Name: | utl_LeaseAssets
Namespace Prefix: | utl_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Lease liabilities. + References No definition available. + Details
Name: | utl_LeaseLiabilities
Namespace Prefix: | utl_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Length of natural gas transmission pipeline. + References No definition available. + Details
Name: | utl_LengthOfPipeline
Namespace Prefix: | utl_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Net gain on divestiture of business. + References No definition available. + Details
Name: | utl_NetGainOnDivestitureOfBusiness
Namespace Prefix: | utl_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Number of Subsidiaries. + References No definition available. + Details
Name: | utl_NumberOfSubsidiaries
Namespace Prefix: | utl_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Percentage Of Sales Volume + References No definition available. + Details
Name: | utl_PercentageOfSalesVolume
Namespace Prefix: | utl_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Significant Accounting Policies [Line Items] + References No definition available. + Details
Name: | utl_SignificantAccountingPoliciesLineItems
Namespace Prefix: | utl_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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

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

X
- Details
Name: | srt_ProductOrServiceAxis=utl_ProductTypeOneMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_ProductOrServiceAxis=utl_ProductTypeTwoMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAssetAxis=utl_EnvironmentalAndRateCaseCostsAndOtherExpendituresMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | utl_PeriodTypeAxis=us-gaap_DebtInstrumentRedemptionPeriodFiveMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_StatementGeographicalAxis=utl_NewEnglandMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_AwardTypeAxis=utl_DeferredCompensationPlanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |