Debt and Financing Arrangements - Additional Information (Detail) - USD ($)		12 Months Ended
	Oct. 04, 2013	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Sep. 30, 2016	Aug. 01, 2016	Oct. 30, 2015	Oct. 15, 2014
Line of Credit Facility [Line Items]
Amount available for the payment of dividends		$ 83,400,000
Retained Earnings		$ 52,200,000	$ 45,100,000
Restriction on retained earnings for dividend payments		Therefore, there were no restrictions on the Company's Retained Earnings at December 31, 2016 for the payment of dividends.
Issuance of long-term debt		$ 300,000
Long term debt repayments		$ 19,000,000	$ 7,400,000	$ 4,400,000
Weighted average interest rate on short term borrowings		1.80%	1.50%	1.60%
Capital lease obligation, current		$ 3,000,000	$ 3,100,000
Capital lease obligation, noncurrent		8,300,000	11,000,000
Accounts Payable		32,400,000	33,300,000
Total rental expense under operating leases		1,800,000	1,700,000	$ 1,300,000
Net Utility Plant		883,400,000	808,900,000
Guarantee outstanding		17,900,000
Assets under Capital Leases
Line of Credit Facility [Line Items]
Net Utility Plant		11,300,000	15,300,000
Net Utility Plant, accumulated amortization		1,000,000	800,000
Financing Arrangements
Line of Credit Facility [Line Items]
Capital lease obligation, current		2,600,000
Capital lease obligation, noncurrent		$ 7,800,000
Unitil Service Corp.
Line of Credit Facility [Line Items]
Capital lease obligation, total capitalized cost							$ 13,400,000
Capital lease obligation, maturity period		Oct. 30, 2015
Capital lease obligation, current		$ 2,600,000
Capital lease obligation, noncurrent		$ 7,800,000
Granite State Gas Transmission Inc
Line of Credit Facility [Line Items]
Total funded indebtedness as percentage of capitalization		65.00%
Northern Utilities Inc
Line of Credit Facility [Line Items]
Total funded indebtedness as percentage of capitalization		65.00%
Natural gas storage inventory		$ 9,900,000	10,800,000
Accounts Payable		$ 2,100,000	600,000
Fitchburg Gas and Electric Light Company
Line of Credit Facility [Line Items]
Total funded indebtedness as percentage of capitalization		65.00%
Unitil Corporation | Maximum
Line of Credit Facility [Line Items]
Total funded indebtedness as percentage of capitalization		70.00%
Unitil Energy, Fitchburg, Northern Utilities and Granite State
Line of Credit Facility [Line Items]
Amount available for the payment of dividends		$ 199,700,000
Revolving Credit Facility
Line of Credit Facility [Line Items]
Revolving credit facility		120,000,000	120,000,000
Proceeds from lines of credit		218,200,000	140,300,000
Repayments of lines of credit		178,300,000	127,600,000
Letters of Credit Outstanding		$ 1,100,000	$ 0		$ 1,100,000
Credit Facility | Revolving Credit Facility
Line of Credit Facility [Line Items]
Revolving credit facility termination date	Oct. 04, 2020
Revolving credit facility	$ 120,000,000
Sublimit for the issuance of standby letters of credit	25,000,000
Credit Facility by an aggregate additional amount	$ 30,000,000
Percentage of capitalization		The affirmative and negative covenants under the Credit Facility shall apply to Unitil until the Credit Facility terminates and all amounts borrowed under the Credit Facility are paid in full (or with respect to letters of credit, they are cash collateralized). The only financial covenant in the Credit Facility provides that Unitil’s Funded Debt to Capitalization (as each term is defined in the Credit Facility) cannot exceed 65%, tested on a quarterly basis. At December 31, 2016 and December 31, 2015, the Company was in compliance with the covenants contained in the Credit Facility in effect on that date.
Credit Facility | Revolving Credit Facility | London Interbank Offered Rate (LIBOR)
Line of Credit Facility [Line Items]
Credit facility, daily fluctuating rate of interest	1.25%
8.00% Senior Secured Notes, Due August 1, 2017 | Unitil Realty Corp
Line of Credit Facility [Line Items]
Long-term debt, stated interest rate		8.00%	8.00%
Long-term debt, maturity date		Aug. 01, 2017	Aug. 01, 2017
Senior Secured Notes		$ 400,000	$ 1,100,000
7.15% Senior Notes, Due December 15, 2018 | Granite State Gas Transmission Inc
Line of Credit Facility [Line Items]
Long-term debt, stated interest rate		7.15%	7.15%
Long-term debt, maturity date		Dec. 15, 2018	Dec. 15, 2018
Senior Notes		$ 6,700,000	$ 10,000,000
3.70% Senior Unsecured Notes due August 1, 2026
Line of Credit Facility [Line Items]
Long-term debt, aggregate principal amount						$ 30,000,000
Long-term debt, stated interest rate		3.70%				3.70%
Long-term debt, maturity date		Aug. 01, 2026
Senior Notes		$ 30,000,000
4.42% Senior Unsecured Notes due October 15, 2044 | Northern Utilities Inc
Line of Credit Facility [Line Items]
Long-term debt, aggregate principal amount								$ 50,000,000
Long-term debt, stated interest rate								4.42%
Long-term debt, maturity date		Oct. 15, 2044
Bonds
Line of Credit Facility [Line Items]
Long term debt repayments		$ 19,000,000	$ 7,400,000	$ 4,400,000
Debt repayment, 2017		17,200,000
Debt repayment, 2018		30,100,000
Debt repayment, 2019		18,800,000
Debt repayment, 2020		19,800,000
Debt repayment, 2021		8,600,000
Debt repayment, Thereafter		$ 242,100,000