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Regulatory Matters - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	1 Months Ended					1 Months Ended					9 Months Ended		1 Months Ended	9 Months Ended				1 Months Ended			9 Months Ended		1 Months Ended
	Aug. 01, 2011 Fitchburg Gas and Electric Light Company Year	Dec. 16, 2011 Fitchburg Gas and Electric Light Company	Nov. 02, 2011 Fitchburg Gas and Electric Light Company Natural Gas Court	Nov. 02, 2009 Fitchburg Gas and Electric Light Company Natural Gas Court	Nov. 02, 2011 Fitchburg Gas and Electric Light Company Natural Gas Supreme Court	Dec. 16, 2011 Fitchburg Gas and Electric Light Company Tropical Storm Irene	Dec. 16, 2011 Fitchburg Gas and Electric Light Company Severe Snow Storm	Aug. 01, 2011 Fitchburg Gas and Electric Light Company Electric Distribution	Aug. 01, 2011 Fitchburg Gas and Electric Light Company Gas Distribution	Aug. 31, 2011 Unitil Energy Systems Inc	Sep. 30, 2012 Unitil Energy Systems Inc	Feb. 28, 2011 Unitil Energy Systems Inc	Dec. 16, 2011 Unitil Energy Systems Inc Storm Costs	Sep. 30, 2012 Unitil Energy Systems Inc Minimum	Sep. 30, 2012 Unitil Energy Systems Inc Maximum	Sep. 30, 2012 Unitil Energy Systems Inc Range One	Sep. 30, 2012 Unitil Energy Systems Inc Range Two	Apr. 26, 2011 Unitil Energy Systems Inc Year One Policy	Apr. 26, 2011 Unitil Energy Systems Inc Year Two Policy	Jun. 29, 2012 Granite State Gas Transmission Inc Gas Transportation and Storage	Sep. 30, 2012 Granite State Gas Transmission Inc Gas Transportation and Storage	Sep. 30, 2012 Granite State Gas Transmission Inc Asset Recoverable Gas Costs	Apr. 24, 2012 Northern Utilities Inc	Mar. 22, 2012 Northern Utilities Inc	Nov. 30, 2011 Northern Utilities Inc Year One Policy	Nov. 30, 2011 Northern Utilities Inc Year Two Policy	Nov. 30, 2011 Northern Utilities Inc Remainder of Year
Regulatory Assets [Line Items]
Increase in annual revenue								$ 3.3	$ 3.7									$ 5.2	$ 5.0						$ 7.8
Return of Equity	9.20%															7.00%	10.00%
Deferred Emergency Storm Restoration Cost	11.4
Amortized and Recovered Cost, Years	7
Additional Cost to Transmission Plant																				2.4	11.4
Amount being Recovered																						1.6
Annual revenue increase in rate adjustments																				0.3
Amendment effective date																		2010-07-01	2011-05-01					2012-05-01	2012-01-01	2012-05-01	2014-01-01
Estimated increases for Capital Improvement														1.5	2.0
Settlement agreement date											May 01, 2016												Apr. 24, 2012
Adjustment to recover the increased spending for vegetation management program and reliability enhancement program											1.47
Increase in annual distribution revenue																										0.85
Increase in annual revenue																								3.7
Refund of Cost			4.6		0.2
Regulatory Liabilities			4.7	4.9
Storm expenditures						1.5	3.3
Portion of storm cost expenditures that the Company has requested deferral of and recovery of through rate adjustments		4.3
Recovery of damage of electrical system													4.4
Interest rate allowed by regulators on the unrecovered amount of storm costs which have been approved for recovery through rate adjustments													4.52%
Recovery of cost, over years													5 years
Customer's Overpayment Amount										1.8
Amount Reimbursed to Customer										1.8
Interest on Reimbursement										0.3
Non-Recurring Charge on Distribution										0.4
Request for recovery of under-billed customers												$ 1.4

X

- Definition

Carrying amount as of the balance sheet date of the portion of the reserved funds established by a utility from customer payments to cover the repair costs of damages from major storms, which repairs are expected to be incurred beyond one year or the normal operating cycle, if longer.

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The increase (decrease) during the reporting period in carrying amount of capitalized electric costs of a regulated entity that are expected to be recoverable through rate adjustments within one year or the normal operating cycle, if longer.

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The increase (decrease) in the evaluation or calculation of the costs that will be required in future periods to make it possible to extract or process proved oil and gas reserves.

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The increase (decrease) during the reporting period in the asset or liability created by a public utility's ability to recover gas costs that are higher or lower than the basic cost included in rate payer calculations.

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The increase (decrease) during the reporting period in unrecovered fuel costs for a regulated utility company.

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Expenses charged against earnings in the period of an event that is infrequent in occurrence or unusual in nature.

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Gas costs of a regulated entity that are refundable through future rate adjustments. Such costs are a form of a regulatory liabilities and are expected to be refunded in less than one year through rate adjustments.

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The amount for the individual regulatory liability as itemized in a table of regulatory liabilities as of the end of the period.

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Repayment received or receivable for expenses incurred on behalf of a client or customer, other than those reimbursements received by landlords from tenants.

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Remaining recovery period of regulatory assets for which no return on investment during recovery period is provided, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Adjustment for rate Increase

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Capital Expenditures

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Distribution Sales Revenue, Goods, Net

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The net change during the reporting period in accrued revenue share.

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Increase Decrease In Costs In Excess Of Billings

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The net change during the reporting period, excluding the portion taken into income, in the liability reflecting revenue yet to be earned for which cash or other forms of consideration was received or recorded as a receivable.

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Represents the interest rate allowed by regulators on the unrecovered amount of storm costs which have been approved for recovery through rate adjustments.

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Interest Received

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Represents the portion of storm cost expenditures that the Company has requested deferral of and recovery of through rate adjustments.

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Rate of Return on Equity Securities

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Regulatory Asset, Recovery Period, Minimum

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Represents an amount for which the company has requested approval from regulators for to recover from customers for underbilling.

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Expenditures related to specific storms.

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