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Regulatory Matters - Additional Information (Detail) (USD $) In Millions, unless otherwise specified | 1 Months Ended | 3 Months Ended |  | 1 Months Ended
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Oct. 31, 2012 Fitchburg Gas and Electric Light Company | May 01, 2012 Fitchburg Gas and Electric Light Company | Feb. 28, 2013 Unitil Energy Systems Inc | Oct. 31, 2012 Unitil Energy Systems Inc | Mar. 31, 2013 Unitil Energy Systems Inc | Mar. 14, 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 | Apr. 30, 2013 Northern Utilities Inc Subsequent Event LegalMatter | Apr. 30, 2013 Northern Utilities Inc New Hampshire Operation Subsequent Event | Apr. 30, 2013 Northern Utilities Inc Maine Operations Subsequent Event
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Increase in annual revenue |  |  |  |  |  |  | $ 5.2 | $ 5.0 | $ 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 |  |  |  |  |  |  |  |
Number of cases filed |  |  |  |  |  |  |  |  | 2 |  |
Percent of increase in annual revenue |  |  |  |  |  |  |  |  |  | 9.40% | 6.30%
Recovery of damage of electrical system | 1.1 |  |  | 2.7 |  |  |  |  |  |  |
Construction Expenditures | 0.3 |  |  | 0.4 |  |  |  |  |  |  |
Storm expenditures |  | 4.3 |  |  |  |  |  |  |  |  |
Recovery of damage of electrical system |  |  |  |  |  | $ 2.3 |  |  |  |  |

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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FASB -Name Statement of Financial Accounting Standard (FAS) -Number 71 -Paragraph 9 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 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 4 -URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4313-108586 Reference 2: 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 Reference 3: 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 + 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 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=6943989&loc=d3e3602-108585 Reference 2: 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. + Details
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