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Regulatory Matters - Additional Information (Detail) (USD $) | 1 Months Ended | 3 Months Ended |  | 1 Months Ended | 3 Months Ended | 1 Months Ended | 3 Months Ended | 1 Months Ended | 3 Months Ended | 1 Months Ended | 3 Months Ended
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Mar. 03, 2014 Unitil Energy Systems Inc | Oct. 31, 2012 Unitil Energy Systems Inc | Mar. 31, 2014 Unitil Energy Systems Inc | Apr. 25, 2013 Unitil Energy Systems Inc | Apr. 25, 2013 Unitil Energy Systems Inc Storm Costs | Jul. 31, 2013 Fitchburg Gas and Electric Light Company | 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 Storm Costs | Feb. 28, 2014 Northern Utilities Inc | Dec. 27, 2013 Northern Utilities Inc | Apr. 21, 2013 Northern Utilities Inc | Mar. 31, 2014 Northern Utilities Inc | Dec. 27, 2013 Northern Utilities Inc Maine | Mar. 31, 2014 Northern Utilities Inc Maine | Apr. 21, 2013 Northern Utilities Inc New Hampshire | Mar. 31, 2014 Northern Utilities Inc New Hampshire | Apr. 21, 2013 Northern Utilities Inc New Hampshire Natural Gas Distribution | Jun. 30, 2013 Granite State Gas Transmission Inc Gas Transportation and Storage | Jun. 30, 2014 Granite State Gas Transmission Inc Gas Transportation and Storage Scenario Forecast
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase in annual revenue |  |  |  |  |  | $ 6,700,000 |  |  | $ 2,100,000 |  |  | $ 1,400,000 | $ 1,000,000 | $ 3,800,000 |  | $ 1,400,000 |  | $ 4,600,000 |  |
Amendment effective date |  |  | May 1, 2014 |  |  |  |  |  |  |  |  |  | May 1, 2014 |  | January 1, 2014 |  | May 1, 2015 |  |  |
TIRA initial term |  |  |  |  |  |  |  |  |  |  |  |  | Four years |  |  |  |  |  |  |
Percentage of eligibility for earnings |  |  |  |  |  |  |  |  |  |  | 10.00% | 10.00% |  |  |  |  |  |  |  |
Percentage of earnings shared equally |  |  |  |  |  |  |  |  |  |  | 11.00% | 11.00% |  |  |  |  |  |  |  |
Service quality plan penalties impossed |  |  |  |  |  |  |  |  |  |  | 500,000 |  |  |  |  |  |  |  |  |
First annual TIRA Adjustment |  |  |  |  |  |  |  |  |  | 1,300,000 |  |  |  |  |  |  |  |  |  |
First annual TIRA Adjustment, effective date |  |  |  |  |  |  |  |  |  |  |  |  | May 1, 2014 |  |  |  |  |  |  |
Percent of increase in annual revenue |  |  |  |  |  | 11.50% |  |  |  |  |  |  |  |  |  | 9.50% |  |  |  |
Adjustment to recover the increased spending for vegetation management program and reliability enhancement program and a proposed storm resiliency program | 1,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Settlement agreement date | May 01, 2016 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Annual revenue increase in rate adjustments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 400,000 | 600,000
Recovery of damage of electrical system |  | 2,700,000 |  |  |  |  | 1,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Construction expenditures |  | 400,000 |  |  |  |  | 300,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Storm expenditures |  |  |  |  |  |  |  | 4,300,000 |  |  |  |  |  |  |  |  |  |  |  |
Recovery of damage of electrical system |  |  |  |  | $ 2,300,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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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