Regulatory Matters - Additional Information (Detail) (USD $)	0 Months Ended			6 Months Ended		0 Months Ended	6 Months Ended	0 Months Ended		6 Months Ended	0 Months Ended	6 Months Ended		0 Months Ended		0 Months Ended	1 Months Ended	6 Months Ended	0 Months Ended	1 Months Ended	0 Months Ended	6 Months Ended
	Feb. 28, 2014 Northern Utilities Inc	Dec. 27, 2013 Northern Utilities Inc	Apr. 21, 2013 Northern Utilities Inc	Jun. 30, 2014 Northern Utilities Inc	Dec. 27, 2013 Northern Utilities Inc	Apr. 21, 2013 Northern Utilities Inc New Hampshire	Jun. 30, 2014 Northern Utilities Inc New Hampshire	Apr. 21, 2013 Northern Utilities Inc New Hampshire Natural Gas Distribution	Dec. 27, 2013 Northern Utilities Inc Maine	Jun. 30, 2014 Northern Utilities Inc Maine	Apr. 30, 2014 Unitil Energy Systems Inc	Jun. 30, 2014 Unitil Energy Systems Inc	Apr. 25, 2013 Unitil Energy Systems Inc	Apr. 25, 2013 Unitil Energy Systems Inc Storm Costs	Apr. 25, 2013 Unitil Energy Systems Inc Storm Costs	May 30, 2014 Fitchburg Gas and Electric Light Company	Jul. 31, 2013 Fitchburg Gas and Electric Light Company	Jun. 30, 2014 Fitchburg Gas and Electric Light Company	May 30, 2014 Fitchburg Gas and Electric Light Company Storm Costs	Jul. 31, 2013 Fitchburg Gas and Electric Light Company Storm Costs	Jun. 27, 2014 Granite State Gas Transmission Inc Gas Transportation and Storage	Jun. 30, 2014 Granite State Gas Transmission Inc Gas Transportation and Storage
Regulatory Matters [Line Items]
Increase in annual revenue			$ 1,400,000	$ 1,000,000		$ 1,400,000		$ 4,600,000	$ 3,800,000							$ 5,600,000	$ 6,700,000
Amendment effective date				May 1, 2014			May 1, 2015			January 1, 2014		May 1, 2014						June 1, 2014				August 1, 2014
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 imposed					500,000
First annual TIRA Adjustment	1,300,000
First annual TIRA Adjustment, effective date				May 1, 2014
Percent of increase in annual revenue						9.50%										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																					600,000
Percentage of approved return on equity																	9.70%
Percentage of approved common equity ratio																	48.00%
Cost recovery period, years														5 years			5 years		3 years	3 years
Deferred emergency storm repair costs incurred																				5,000,000
Annual funding amount approved																	500,000
Recovery amount															2,300,000		900,000
Storm expenditures																			$ 5,000,000
Interest rate allowed by regulators on the unrecovered amount of storm costs which have been approved for recovery through rate adjustments													4.52%