Common Stock and Preferred Stock - Additional Information (Detail) - USD ($)
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3 Months Ended |
6 Months Ended |
12 Months Ended |
Apr. 19, 2016 |
Jan. 26, 2016 |
Jun. 30, 2016 |
Jun. 30, 2015 |
Jun. 30, 2016 |
Jun. 30, 2015 |
Dec. 31, 2015 |
| Class of Stock [Line Items] |
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| Common stock, shares outstanding |
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14,052,096
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13,974,576
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14,052,096
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13,974,576
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13,991,430
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| Common stock, shares issued |
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16,646
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19,411
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| Proceeds from Issuance of Common Stock |
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$ 700,000
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$ 600,000
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| Share based compensation expense |
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$ 1,800,000
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1,500,000
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| Percentage of fully-vested restricted stock units that directors will receive in common shares when settled |
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70.00%
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| Percentage of fully-vested restricted stock units that directors will receive in cash when settled |
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30.00%
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| Fair value of liabilities associated with fully vested RSUs that will be settled in cash |
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$ 600,000
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$ 300,000
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$ 600,000
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300,000
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$ 500,000
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| Preferred Stock |
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200,000
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200,000
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200,000
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200,000
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$ 200,000
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| Maximum |
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| Class of Stock [Line Items] |
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| Dividend declared |
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$ 100,000
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$ 100,000
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100,000
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$ 100,000
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| Dividend and Distribution Reinvestment and Share Purchase Plan |
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| Class of Stock [Line Items] |
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| Proceeds from Issuance of Common Stock |
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$ 659,000
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| Dividend and Distribution Reinvestment and Share Purchase Plan | Average |
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| Class of Stock [Line Items] |
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| Common stock price per share |
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$ 39.58
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$ 39.58
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| Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock |
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| Class of Stock [Line Items] |
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| Common stock, shares issued |
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16,646
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| Unitil Energy Systems Inc | Series 6 |
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| Class of Stock [Line Items] |
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| Preferred stock, outstanding |
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1,898
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2,059
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1,898
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2,059
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1,898
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| Preferred Stock |
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$ 200,000
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$ 200,000
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$ 200,000
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$ 200,000
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$ 200,000
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| Dividend rate |
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6.00%
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6.00%
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6.00%
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| Restricted Stock |
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| Class of Stock [Line Items] |
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| Restricted stock vesting period |
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4 years
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| Restricted stock non-vested |
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95,506
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80,588
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95,506
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80,588
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| Restricted stock weighted average grant date fair value |
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$ 35.30
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$ 33.14
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| Unrecognized share based compensation |
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$ 1,600,000
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$ 1,600,000
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| Share compensation recognition period |
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2 years 8 months 12 days
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| Forfeitures or cancellations under the stock plan |
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0
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| Restricted Shares issued |
800
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43,220
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| Aggregate Market Value |
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$ 1,600,000
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| Restricted Stock | Maximum |
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| Class of Stock [Line Items] |
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| Restricted stock available for awards |
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677,500
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677,500
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| Restricted stock that may be awarded in any one calendar year to any one participant |
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20,000
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20,000
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| Aggregate Market Value |
$ 100,000
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| Restricted Stock | Vesting Annually |
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| Class of Stock [Line Items] |
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| Restricted stock vesting percentage annually |
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25.00%
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| Restricted Stock Units (RSUs) |
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| Class of Stock [Line Items] |
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| Restricted Shares issued |
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0
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| Restricted stock units outstanding |
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24,062
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24,062
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| Weighted-Average Stock Price |
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$ 29.98
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$ 29.98
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