Carrying value as of the balance sheet date of liabilities incurred (and for which invoices have typically been received) and payable to vendors for goods and services received that are used in an entity's business. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer).

Name:	us-gaap_AccountsPayableCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Debt and Financing Arrangements - Additional Information (Detail) - USD ($)			9 Months Ended
	Jul. 14, 2017	Oct. 04, 2013	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Oct. 30, 2015
Line of Credit Facility [Line Items]
Weighted average interest rate on short term borrowings			2.30%	1.80%	1.70%
Deferred financing cost			$ 100,000
Capital lease obligation, current			3,100,000	$ 3,000,000	$ 3,100,000
Capital lease obligation, noncurrent			6,400,000	8,300,000	9,000,000
Accounts Payable			24,100,000	32,400,000	23,100,000
Guarantee outstanding			$ 19,800,000
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			$ 2,700,000
Capital lease obligation, noncurrent			5,800,000
Northern Utilities Inc
Line of Credit Facility [Line Items]
Natural gas storage inventory			9,000,000	9,900,000	9,300,000
Accounts Payable			100,000	2,100,000	100,000
Revolving Credit Facility
Line of Credit Facility [Line Items]
Revolving credit facility			120,000,000	$ 120,000,000	$ 120,000,000
Proceeds from lines of credit			164,200,000
Repayments of lines of credit			$ 134,200,000
3.52% Senior Unsecured Notes, Due 2027 \| Northern Utilities Inc
Line of Credit Facility [Line Items]
Senior Notes	$ 20,000,000
Debt instrument due date, year	2027
Stated interest rate	3.52%
3.52% Senior Unsecured Notes, Due 2027 \| Fitchburg Gas and Electric Light Company
Line of Credit Facility [Line Items]
Senior Notes	$ 10,000,000
Debt instrument due date, year	2027
Stated interest rate	3.52%
4.32% Senior Unsecured Notes, Due 2047 \| Northern Utilities Inc
Line of Credit Facility [Line Items]
Senior Notes	$ 30,000,000
Debt instrument due date, year	2047
Stated interest rate	4.32%
4.32% Senior Unsecured Notes, Due 2047 \| Fitchburg Gas and Electric Light Company
Line of Credit Facility [Line Items]
Senior Notes	$ 15,000,000
Debt instrument due date, year	2047
Stated interest rate	4.32%
3.72% Senior Unsecured Notes, Due 2027 \| Granite State Gas Transmission Inc
Line of Credit Facility [Line Items]
Senior Notes	$ 15,000,000
Debt instrument due date, year	2027
Stated interest rate	3.72%
Credit Facility \| Revolving Credit Facility
Line of Credit Facility [Line Items]
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
Revolving credit facility termination date		Oct. 04, 2020
Percentage of capitalization			The affirmative and negative covenants under the Credit Facility shall apply 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 Funded Debt to Capitalization (as each term is defined in the Credit Facility) cannot exceed 65%, tested on a quarterly basis. At September 30, 2017, September 30, 2016 and December 31, 2016, 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%
Senior Unsecured Notes \| Private Placement
Line of Credit Facility [Line Items]
Senior Notes	$ 90,000,000
7.15% Senior Notes, Due December 15, 2018 \| Granite State Gas Transmission Inc
Line of Credit Facility [Line Items]
Senior Notes			$ 6,700,000
Stated interest rate			7.15%	7.15%	7.15%