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
	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%