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.

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Term of lessee's operating lease, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period.

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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).

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Gain on divestiture of business pretax.

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Lease assets.

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Lease liabilities.

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Length of natural gas transmission pipeline.

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Net gain on divestiture of business.

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Number of Subsidiaries.

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Significant Accounting Policies [Line Items]

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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