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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Future cash outflow to pay for construction in progress expenditures that have occurred.

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Carrying amount as of the balance sheet date of capitalized storm costs of a regulated entity that are expected to be recoverable through rate adjustments after one year or beyond the normal operating cycle, if longer.

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

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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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A noncash adjustment to the provision established for storm damage by an oil, gas, or utility company.

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Adjustment for rate Increase

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Amendment Effective Date

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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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Percentage Of Revenue Increase

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Settlement Dates

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Expenditures related to specific storms.

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Regulatory Matters - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	1 Months Ended	9 Months Ended	1 Months Ended
	Oct. 31, 2012 Unitil Energy Systems Inc	Sep. 30, 2013 Unitil Energy Systems Inc	Apr. 25, 2013 Unitil Energy Systems Inc Storm Costs	Apr. 26, 2011 Unitil Energy Systems Inc Year One Policy	Apr. 26, 2011 Unitil Energy Systems Inc Year Two Policy	Oct. 31, 2012 Fitchburg Gas and Electric Light Company	May 01, 2012 Fitchburg Gas and Electric Light Company	Jul. 31, 2013 Fitchburg Gas and Electric Light Company Subsequent Event	Jul. 31, 2013 Fitchburg Gas and Electric Light Company Subsequent Event Storm Costs	Jun. 30, 2013 Granite State Gas Transmission Inc Gas Transportation and Storage	Apr. 30, 2013 Northern Utilities Inc	Apr. 30, 2013 Northern Utilities Inc New Hampshire	Apr. 30, 2013 Northern Utilities Inc Maine
Regulatory Assets [Line Items]
Annual revenue increase in rate adjustments										$ 0.4
Increase in annual revenue				5.2	5.0			6.7	2.1		2.5	5.2	4.6
Amendment effective date				July 1, 2010	May 1, 2011						July 1, 2013
Settlement agreement date		May 01, 2016
Adjustment to recover the increased spending for vegetation management program and reliability enhancement program and a proposed storm resiliency program		2.8
Percent of increase in annual revenue								11.50%				9.40%	6.30%
Storm recovery adjustment factor amount								2.8
Recovery of damage of electrical system	2.7					1.1
Construction expenditures	0.4					0.3
Storm expenditures							4.3
Recovery of damage of electrical system			$ 2.3
Cost recovery period, years			5 years
Interest rate allowed by regulators on the unrecovered amount of storm costs which have been approved for recovery through rate adjustments			4.52%