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Regulatory Matters - Additional Information (Detail) - USD ($) $ in Millions							6 Months Ended	9 Months Ended	12 Months Ended
	Jun. 16, 2015	Apr. 15, 2015	Nov. 26, 2014	Apr. 30, 2014	Apr. 21, 2014	Dec. 27, 2013	Jun. 30, 2015	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Regulatory Matters [Line Items]
Current Portion of Regulatory Liabilities								$ 28.1	$ 8.7	$ 13.0
Regulatory Liabilities - noncurrent								$ 11.0	0.1
Northern Utilities Inc
Regulatory Matters [Line Items]
TIRA initial term								Four years
Pipeline refund received		$ 22.0
Current Portion of Regulatory Liabilities								$ 11.1
Regulatory Liabilities - noncurrent								$ 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 | Northern Utilities' customers
Regulatory Matters [Line Items]
Settlement agreement date								Oct. 05, 2015
Northern Utilities Inc | Maine
Regulatory Matters [Line Items]
Increase in annual revenue						$ 3.8
Amendment effective date								Jan. 01, 2014
Northern Utilities Inc | Maine | Northern Utilities' customers
Regulatory Matters [Line Items]
Grace period to issue refund								3 years
Northern Utilities Inc | New Hampshire
Regulatory Matters [Line Items]
Increase in annual revenue								$ 1.8
Amendment effective date								May 01, 2015
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 | New Hampshire | Northern Utilities' customers
Regulatory Matters [Line Items]
Grace period to issue refund								3 years
Unitil Energy Systems Inc
Regulatory Matters [Line Items]
Amendment effective date								May 01, 2014
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				$ 1.5
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
Fitchburg Gas and Electric Light Company
Regulatory Matters [Line Items]
Amendment effective date								May 01, 2016
Storm expenditures			0.2
Construction expenditures			0.1
Recovery amount								$ 0.3
Fitchburg Gas and Electric Light Company | Storm Costs
Regulatory Matters [Line Items]
Storm expenditures			$ 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%
Granite State Gas Transmission Inc | Gas Transportation and Storage
Regulatory Matters [Line Items]
Increase in annual revenue							$ 0.4
Amendment effective date							Aug. 01, 2015

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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Future cash outflow to pay for construction in progress expenditures that have occurred.

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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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