Summary of Significant Accounting Policies - Additional Information (Detail) $ / shares in Units, Bcf in Billions		1 Months Ended	3 Months Ended								12 Months Ended
	Dec. 31, 2015 USD ($) Bcf	Feb. 02, 2017 $ / shares	Dec. 31, 2016 USD ($) $ / shares Bcf	Sep. 30, 2016 $ / shares	Jun. 30, 2016 $ / shares	Mar. 31, 2016 $ / shares	Dec. 31, 2015 USD ($) $ / shares Bcf	Sep. 30, 2015 $ / shares	Jun. 30, 2015 $ / shares	Mar. 31, 2015 $ / shares	Dec. 31, 2016 USD ($) Entity Subsidiary mi $ / shares Bcf	Dec. 31, 2015 USD ($) $ / shares Bcf	Dec. 31, 2014 $ / shares
Significant Accounting Policies [Line Items]
Number of Subsidiaries | Entity											3
Length Of Pipeline | mi											86
Depreciation rate based on average depreciable property balance											3.49%	3.57%	3.56%
Common stock dividend paid per share | $ / shares			$ 0.355	$ 0.355	$ 0.355	$ 0.355	$ 0.350	$ 0.350	$ 0.350	$ 0.350	$ 1.42	$ 1.40	$ 1.38
Common stock dividend per share, declared | $ / shares											$ 1.42	$ 1.40	$ 1.38
Average interest rates											2.18%	2.32%	1.56%
Cost of removal obligation	$ 70,100,000		$ 77,000,000				$ 70,100,000				$ 77,000,000	$ 70,100,000
Regulatory Assets	$ 126,400,000		$ 142,000,000				$ 126,400,000				$ 142,000,000	$ 126,400,000
Number of Natural Gas Storage Outstanding | Bcf	2.5		2.0				2.5				2.0	2.5
Investments in trading securities	$ 700,000		$ 1,900,000				$ 700,000				$ 1,900,000	$ 700,000
Intangible Assets, Purchase Adjustments											$ 4,700,000
Reduction of other assets	(2,900,000)
Reduction of long-term debt	(2,900,000)
Plant
Significant Accounting Policies [Line Items]
Intangible Asset Amortization Period											10 years
Bargain Purchase Adjustment
Significant Accounting Policies [Line Items]
Intangible Asset Amortization Period											2 years
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	2,300,000		2,800,000				2,300,000				$ 2,800,000	2,300,000
Subsequent Event
Significant Accounting Policies [Line Items]
Common stock dividend per share, declared | $ / shares		$ 1.44
Natural Gas Hedging Program
Significant Accounting Policies [Line Items]
Cash Deposits	0		0				0				$ 0	$ 0
Quarterly Dividends
Significant Accounting Policies [Line Items]
Common stock dividend paid per share | $ / shares											$ 0.355	$ 0.350	$ 0.345
Quarterly Dividends | Subsequent Event
Significant Accounting Policies [Line Items]
Common stock dividend per share, declared | $ / shares		0.36
Increase in dividend declared amount per share | $ / shares		$ 0.005
Deferred Storm Charges
Significant Accounting Policies [Line Items]
Regulatory Assets	$ 15,400,000		9,600,000				$ 15,400,000				$ 9,600,000	$ 15,400,000
Deferred Storm Charges | Period 4
Significant Accounting Policies [Line Items]
Regulatory Assets			3,300,000								3,300,000
Environmental and Rate Case Costs and Other Expenditures | Recovered over the next seven and a half years
Significant Accounting Policies [Line Items]
Regulatory Assets			$ 8,600,000								$ 8,600,000
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