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Common Stock and Preferred Stock - Additional Information (Detail) - USD ($) |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Jan. 26, 2016 | Mar. 31, 2016 | Mar. 31, 2015 | Dec. 31, 2015
Class of Stock [Line Items] |  |  |  |
Common stock, shares outstanding |  | 14,043,280 | 13,966,150 | 13,991,430
Common stock, shares issued |  | 8,630 | 10,114 |
Proceeds from Issuance of Common Stock |  | $ 300,000 | $ 300,000 |
Share based compensation expense |  | $ 1,600,000 | 1,300,000 |
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 |  | $ 600,000 | 400,000 | $ 500,000
Preferred Stock |  | 200,000 | 200,000 | $ 200,000
Maximum |  |  |  |
Class of Stock [Line Items] |  |  |  |
Dividend declared |  | 100,000 | $ 100,000 |
Dividend and Distribution Reinvestment and Share Purchase Plan |  |  |  |
Class of Stock [Line Items] |  |  |  |
Proceeds from Issuance of Common Stock |  | $ 338,000 |  |
Dividend and Distribution Reinvestment and Share Purchase Plan | Average |  |  |  |
Class of Stock [Line Items] |  |  |  |
Common stock price per share |  | $ 39.19 |  |
Dividend and Distribution Reinvestment and Share Purchase Plan | Common Stock |  |  |  |
Class of Stock [Line Items] |  |  |  |
Common stock, shares issued |  | 8,630 |  |
Unitil Energy Systems Inc | Series 6 |  |  |  |
Class of Stock [Line Items] |  |  |  |
Preferred stock, outstanding |  | 1,898 | 2,250 | 1,898
Preferred Stock |  | $ 200,000 | $ 200,000 | $ 200,000
Dividend rate |  | 6.00% | 6.00% | 6.00%
Restricted Stock |  |  |  |
Class of Stock [Line Items] |  |  |  |
Restricted stock vesting period |  | 4 years |  |
Restricted stock non-vested |  | 94,706 | 81,251 |
Restricted stock weighted average grant date fair value |  | $ 35.24 | $ 33.15 |
Unrecognized share based compensation |  | $ 1,700,000 |  |
Share compensation recognition period |  | 2 years 10 months 24 days |  |
Forfeitures or cancellations under the stock plan |  | 0 |  |
Restricted Shares issued | 43,220 |  |  |
Aggregate Market Value | $ 1,600,000 |  |  |
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 |  |
Restricted Stock | Vesting Annually |  |  |  |
Class of Stock [Line Items] |  |  |  |
Restricted stock vesting percentage annually |  | 25.00% |  |
Restricted Stock Units (RSUs) |  |  |  |
Class of Stock [Line Items] |  |  |  |
Restricted Shares issued |  | 0 |  |
Restricted stock units outstanding |  |  | 23,819 |
Weighted-Average Stock Price |  |  | $ 29.94 |

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