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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
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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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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- 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 71 -Paragraph 9 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499739&loc=d3e43603-110378 + Details
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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 28 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585 + Details
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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 19 -Paragraph 59BB -Subparagraph b -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 31 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e62455-109447 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 19 -Paragraph 59Y -Subparagraph b -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 35 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e62500-109447 + Details
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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 28 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585 + Details
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- Definition The increase (decrease) during the reporting period in unrecovered fuel costs for a regulated utility company. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 28 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585 + Details
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- Definition Expenses charged against earnings in the period of an event that is infrequent in occurrence or unusual in nature. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 20 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6365513&loc=d3e15122-107781 + Details
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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.20) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 71 -Paragraph 11 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 405 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6500807&loc=d3e48068-110394 + Details
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- Definition The amount for the individual regulatory liability as itemized in a table of regulatory liabilities as of the end of the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 71 -Paragraph 11 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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- Definition Repayment received or receivable for expenses incurred on behalf of a client or customer, other than those reimbursements received by landlords from tenants. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.1) -URI http://asc.fasb.org/extlink&oid=6880815&loc=d3e20235-122688 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 605 -SubTopic 45 -Section 45 -Paragraph 23 -URI http://asc.fasb.org/extlink&oid=21915142&loc=d3e60722-111653 + Details
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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 55 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6591318&loc=d3e44360-110383 + Details
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- Definition Adjustment for rate Increase + References No definition available. + Details
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- Definition Capital Expenditures + References No definition available. + Details
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- Definition The net change during the reporting period in accrued revenue share. + References No definition available. + Details
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- Definition Increase Decrease In Costs In Excess Of Billings + References No definition available. + Details
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- Definition 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. + References No definition available. + Details
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- Definition Represents the interest rate allowed by regulators on the unrecovered amount of storm costs which have been approved for recovery through rate adjustments. + References No definition available. + Details
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- Definition Interest Received + References No definition available. + Details
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- Definition Represents the portion of storm cost expenditures that the Company has requested deferral of and recovery of through rate adjustments. + References No definition available. + Details
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- Definition Regulatory Asset, Recovery Period, Minimum + References No definition available. + Details
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- Definition Represents an amount for which the company has requested approval from regulators for to recover from customers for underbilling. + References No definition available. + Details
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- Definition Expenditures related to specific storms. + References No definition available. + Details
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