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Summary of Significant Accounting Policies - Additional Information (Detail) $ / shares in Units, Bcf in Billions	1 Months Ended	3 Months Ended								12 Months Ended
	Feb. 02, 2018 $ / shares	Dec. 31, 2017 USD ($) $ / shares Bcf	Sep. 30, 2017 $ / shares	Jun. 30, 2017 $ / shares	Mar. 31, 2017 $ / shares	Dec. 31, 2016 USD ($) $ / shares Bcf	Sep. 30, 2016 $ / shares	Jun. 30, 2016 $ / shares	Mar. 31, 2016 $ / shares	Dec. 31, 2017 USD ($) Entity Subsidiary mi $ / shares Bcf	Dec. 31, 2016 USD ($) $ / shares Bcf	Dec. 31, 2015 $ / shares
Significant Accounting Policies [Line Items]
Number of Subsidiaries | Entity										3
Length Of Pipeline | mi										86
Depreciation rate based on average depreciable property balance										3.45%	3.49%	3.57%
Common stock dividend paid per share | $ / shares		$ 0.360	$ 0.360	$ 0.360	$ 0.360	$ 0.355	$ 0.355	$ 0.355	$ 0.355	$ 1.44	$ 1.42	$ 1.40
Common stock dividend per share, declared | $ / shares										$ 1.44	$ 1.42	$ 1.40
Average interest rates										2.90%	2.18%	2.32%
Cost of removal obligation		$ 84,300,000				$ 77,000,000				$ 84,300,000	$ 77,000,000
Regulatory Assets		$ 149,100,000				$ 142,000,000				$ 149,100,000	$ 142,000,000
Number of Natural Gas Storage Outstanding | Bcf		0.6				2.0				0.6	2.0
Investments in trading securities		$ 3,600,000				$ 1,900,000				$ 3,600,000	$ 1,900,000
Intangible Assets, Purchase Adjustments										$ 2,300,000
Plant
Significant Accounting Policies [Line Items]
Intangible Asset Amortization Period										10 years
Bargain Purchase Adjustment
Significant Accounting Policies [Line Items]
Intangible Asset Amortization Period										1 year
Subsequent Event
Significant Accounting Policies [Line Items]
Common stock dividend per share, declared | $ / shares	$ 1.46
Quarterly Dividends
Significant Accounting Policies [Line Items]
Common stock dividend paid per share | $ / shares										$ 0.360	$ 0.355	$ 0.350
Quarterly Dividends | Subsequent Event
Significant Accounting Policies [Line Items]
Common stock dividend per share, declared | $ / shares	0.365
Increase in dividend declared amount per share | $ / shares	$ 0.005
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%
Maximum
Significant Accounting Policies [Line Items]
Cash equivalents maturity period										3 months
ISO-NE Obligations
Significant Accounting Policies [Line Items]
Cash Deposits		2,900,000				2,800,000				$ 2,900,000	$ 2,800,000
Natural Gas Hedging Program
Significant Accounting Policies [Line Items]
Cash Deposits		0				0				0	0
Deferred Storm Charges
Significant Accounting Policies [Line Items]
Regulatory Assets		7,200,000				$ 9,600,000				7,200,000	$ 9,600,000
Deferred Storm Charges | Recovered over Next Year
Significant Accounting Policies [Line Items]
Regulatory Assets		900,000								900,000
Environmental and Rate Case Costs and Other Expenditures | Recovered over the next seven years
Significant Accounting Policies [Line Items]
Regulatory Assets		$ 7,600,000								$ 7,600,000
Unitil Service; Unitil Realty; and Unitil Resources
Significant Accounting Policies [Line Items]
Number of Subsidiaries | Subsidiary										3
Utilities
Significant Accounting Policies [Line Items]
Number of Subsidiaries | Subsidiary										3

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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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