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Common Stock and Preferred Stock - Additional Information (Detail) (USD $)	3 Months Ended		0 Months Ended	12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Jan. 26, 2015	Dec. 31, 2014
Class of Stock [Line Items]
Common stock, shares outstanding	13,966,150us-gaap_CommonStockSharesOutstanding	13,886,768us-gaap_CommonStockSharesOutstanding		13,916,026us-gaap_CommonStockSharesOutstanding
Common stock, shares issued	10,114	9,868
Proceeds from Issuance of Common Stock	$ 300,000	$ 300,000
Share based compensation expense	1,300,000us-gaap_RestrictedStockExpense	1,100,000us-gaap_RestrictedStockExpense
Preferred Stock	200,000us-gaap_PreferredStockValue	200,000us-gaap_PreferredStockValue		200,000us-gaap_PreferredStockValue
Maximum
Class of Stock [Line Items]
Dividend declared	100,000	100,000
Restricted Stock Units
Class of Stock [Line Items]
Percentage of fully-vested restricted stock units that directors will receive in common shares when settled	70.00%
Percentage of fully-vested restricted stock units that directors will receive in cash when settled	30.00%
Fair value of liabilities associated with fully vested RSUs that will be settled in cash	400,000	200,000		400,000
Restricted stock units outstanding		15,066
Weighted-Average Stock Price		$ 28.93
Dividend and Distribution Reinvestment and Share Purchase Plan
Class of Stock [Line Items]
Common stock price per share	$ 34.27
Proceeds from Issuance of Common Stock	347,000
Dividend and Distribution Reinvestment and Share Purchase Plan \| Common Stock
Class of Stock [Line Items]
Common stock, shares issued	10,114
Restricted Stock
Class of Stock [Line Items]
Restricted stock vesting period	4 years
Restricted stock vesting percentage annually	25.00%
Restricted Stock Units Granted	0		40,010
Aggregate Market Value			1,500,000
Restricted stock non-vested	81,251	69,073
Restricted stock weighted average grant date fair value	$ 33.15	$ 28.52
Unrecognized share based compensation	1,500,000
Share compensation recognition period	3 years
Forfeitures or cancellations under the stock plan	0
Restricted Stock \| Maximum
Class of Stock [Line Items]
Restricted stock available for awards	677,500
Restricted stock that may be awarded in any one calendar year to any one participant	20,000
Unitil Energy Systems Inc \| Series 6
Class of Stock [Line Items]
Preferred stock, outstanding	2,250	2,250		2,250
Preferred Stock	$ 200,000	$ 200,000		$ 200,000
Dividend rate	6.00%	6.00%		6.00%