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Common Stock and Preferred Stock - Additional Information (Detail) (USD $) | 3 Months Ended |  | 3 Months Ended |  |  |  | 3 Months Ended |  | 12 Months Ended |  | 12 Months Ended |  | 1 Months Ended | 3 Months Ended |  |  |
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Mar. 31, 2013 | Mar. 31, 2012 | Dec. 31, 2012 | Mar. 31, 2013 Maximum | Mar. 31, 2012 Maximum | Mar. 31, 2012 Restricted Stock Unit | Mar. 31, 2013 Restricted Stock Unit Maximum | Dec. 31, 2012 Restricted Stock Unit Maximum | Mar. 31, 2013 Restricted Stock Y | Mar. 31, 2012 Restricted Stock | Mar. 31, 2013 Restricted Stock Maximum | Dec. 31, 2012 Series 8 Fitchburg Gas and Electric Light Company | Mar. 31, 2012 Series 8 Fitchburg Gas and Electric Light Company | Dec. 31, 2012 Series 5 Fitchburg Gas and Electric Light Company | Mar. 31, 2012 Series 5 Fitchburg Gas and Electric Light Company | May 16, 2012 Common Stock Offering | Mar. 31, 2013 Dividend and Distribution Reinvestment and Share Purchase Plan | Mar. 31, 2013 Restricted Stock Units Issued | Mar. 31, 2013 Series 6 Unitil Energy Systems Inc | Dec. 31, 2012 Series 6 Unitil Energy Systems Inc | Mar. 31, 2012 Series 6 Unitil Energy Systems Inc
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, shares outstanding | 13,812,763 | 10,989,389 | 13,780,601 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, share issued |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2,760,000 | 10,922 |  |  |  |
Common stock price per share |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 25.25 | $ 27.01 |  |  |  |
Proceed from issuance of common stock |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 65,700,000 | $ 295,000 |  |  |  |
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 Period |  |  |  |  |  |  |  |  | 4 years |  |  |  |  |  |  |  |  |  |  |  |
Restricted stock Vesting Percentage Annually |  |  |  |  |  |  |  |  | 25.00% |  |  |  |  |  |  |  |  |  |  |  |
Restricted stock issue date |  |  |  |  |  |  |  |  | Feb. 04, 2013 |  |  |  |  |  |  |  |  |  |  |  |
Restricted stock issued |  |  |  |  |  |  |  |  | 21,240 |  |  |  |  |  |  |  |  |  |  |  |
Restricted stock grant date market value |  |  |  |  |  |  |  |  | 564,134 |  |  |  |  |  |  |  |  |  |  |  |
Restricted stock non-vested |  |  |  |  |  |  |  |  | 53,480 | 57,638 |  |  |  |  |  |  |  |  |  |  |
Restricted stock weighted average grant date fair value |  |  |  |  |  |  |  |  | $ 25.99 | $ 24.68 |  |  |  |  |  |  |  |  |  |  |
Share based compensation expense |  |  |  |  |  |  |  |  | 400,000 | 300,000 |  |  |  |  |  |  |  |  |  |  |
Unrecognized share based compensation |  |  |  |  |  |  |  |  | 1,000,000 |  |  |  |  |  |  |  |  |  |  |  |
Share compensation recognition period |  |  |  |  |  |  |  |  | 3.0 |  |  |  |  |  |  |  |  |  |  |  |
Forfeitures or cancellations under the stock plan |  |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |  |  |  |
Restricted stock units issued |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0 |  |  |
Fair value of liabilities associated with fully vested RSUs that will be settled in cash |  |  |  |  |  | 0 | 100,000 | 100,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred stock, outstanding |  |  |  |  |  |  |  |  |  |  |  |  | 9,696 |  | 7,861 |  |  |  | 2,250 | 2,250 | 2,250
Redeemable, Cumulative Preferred Stock |  |  |  |  |  |  |  |  |  |  |  | 965,400 |  | 792,313 |  |  |  |  |  |  |
Dividend rate |  |  |  |  |  |  |  |  |  |  |  | 8.00% |  | 5.125% |  |  |  |  |  |  |
Preferred Stock premium percentage |  |  |  |  |  |  |  |  |  |  |  |  |  | 1.28% |  |  |  |  |  |  |
Dividend declared |  |  |  | $ 100,000 | $ 100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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