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Regulatory Matters - Additional Information (Detail) - USD ($) $ in Millions											1 Months Ended	6 Months Ended	12 Months Ended
	Jun. 16, 2015	Apr. 15, 2015	Feb. 27, 2015	Feb. 19, 2015	Nov. 26, 2014	Jun. 27, 2014	Apr. 30, 2014	Apr. 21, 2014	Dec. 27, 2013	May. 12, 2010	Jun. 30, 2015	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Regulatory Matters [Line Items]
Current Portion of Regulatory Liabilities											$ 31.2	$ 31.2	$ 8.7	$ 13.0
Regulatory Liabilities - noncurrent											11.0	11.0	0.1
Fitchburg Gas and Electric Light Company
Regulatory Matters [Line Items]
Storm expenditures					$ 0.2
Construction expenditures					0.1
Recovery amount											0.3	$ 0.3
Fitchburg Gas and Electric Light Company | Electric base
Regulatory Matters [Line Items]
Increase in annual revenue	$ 3.8
Percentage of approved return on equity	5.60%
Fitchburg Gas and Electric Light Company | Gas base
Regulatory Matters [Line Items]
Increase in annual revenue	$ 3.0
Percentage of approved return on equity	8.30%
Fitchburg Gas and Electric Light Company | Storm Costs
Regulatory Matters [Line Items]
Storm expenditures					0.3
Unitil Energy Systems Inc
Regulatory Matters [Line Items]
Amendment effective date												May 01, 2014
Adjustment to recover the increased spending for vegetation management program and reliability enhancement program and a proposed storm resiliency program							$ 1.5
Settlement agreement date												May 01, 2016
Construction expenditures					0.4
Recovery of damage of electrical system					1.7
Unitil Energy Systems Inc | Storm Costs
Regulatory Matters [Line Items]
Storm expenditures					$ 2.1
Northern Utilities Inc
Regulatory Matters [Line Items]
TIRA initial term												Four years
Proposed percentage increase in pipeline rates										55.00%
Pipeline refund received		$ 22.0
Current Portion of Regulatory Liabilities											11.1	$ 11.1
Regulatory Liabilities - noncurrent											10.9	$ 10.9
Northern Utilities Inc | First Annual Adjustment
Regulatory Matters [Line Items]
Annual TIRA Adjustment													$ 1.3
Annual TIRA Adjustment, effective date												May 01, 2014
Northern Utilities Inc | Second Annual Adjustment
Regulatory Matters [Line Items]
Annual TIRA Adjustment			$ 1.2
Annual TIRA Adjustment, effective date												May 01, 2015
Northern Utilities Inc | 2014 Step Adjustments
Regulatory Matters [Line Items]
Increase in annual revenue								$ 1.4
Amendment effective date												May 01, 2014
Northern Utilities Inc | Maine
Regulatory Matters [Line Items]
Increase in annual revenue									$ 3.8
Amendment effective date												Jan. 01, 2014
Northern Utilities Inc | New Hampshire
Regulatory Matters [Line Items]
Increase in annual revenue			$ 1.8
Amendment effective date												May 01, 2015
Percent of increase in annual revenue								9.50%
Northern Utilities Inc | New Hampshire | Natural Gas Distribution
Regulatory Matters [Line Items]
Increase in annual revenue								$ 4.6
Amendment effective date												May 01, 2014
Northern Utilities Inc | Maximum | Portland Natural Gas Transmission System
Regulatory Matters [Line Items]
Grace period to issue refund				60 days
Northern Utilities Inc | Maximum | Northern Utilities' customers
Regulatory Matters [Line Items]
Grace period to issue refund												3 years
Granite State Gas Transmission Inc | Gas Transportation and Storage
Regulatory Matters [Line Items]
Increase in annual revenue											$ 0.4
Amendment effective date												Aug. 01, 2014
Annual revenue increase in rate adjustments						$ 0.6

X

- 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.

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

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Percentage of public utility's approved return on equity.

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The amount for the individual regulatory current liability as itemized in a table of regulatory current liabilities as of the end of the period.

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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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Expenditures related to specific storms.

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