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Regulatory Matters - Additional Information (Detail) (USD $) In Millions, unless otherwise specified | 1 Months Ended | 9 Months Ended | 1 Months Ended
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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% |  |  |  |  |  |  |  |  |  |

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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 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 Future cash outflow to pay for construction in progress expenditures that have occurred. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4332-108586 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4313-108586 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4304-108586 + 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 Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 + 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 A noncash adjustment to the provision established for storm damage by an oil, gas, or utility company. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 + Details
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