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Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions | 3 Months Ended |  |  |
--- | --- | --- | --- | ---
Mar. 31, 2019 USD ($) Subsidiary mi | Jan. 01, 2019 USD ($) | Dec. 31, 2018 USD ($) | Mar. 31, 2018 USD ($)
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 | 93.7 |  | $ 90.7 | $ 86.6
Recognition of lease assets |  | $ 4.2 |  |
Recognition of lease liabilities |  | $ 4.2 |  |
Regulatory assets | 127.3 |  | 140.3 | 145.8
Investments in trading securities | 5.1 |  | 4.8 | 5.1
Deferred Compensation Plan [Member] |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |
Investments in trading securities | $ 0.1 |  | 0.0 | 0.0
Maximum |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |
Cash equivalents maturity period | 3 months |  |  |
Lease term | 12 months |  |  |
ISO-NE Obligations |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |
Cash Deposits | $ 3.0 |  | $ 3.5 | $ 3.3
Environmental and Rate Case Costs and Other Expenditures | Recovered over the next seven years |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |
Regulatory assets | $ 5.9 |  |  |
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 |  |  |

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- 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
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- 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
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- 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
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- 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
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