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Summary of Significant Accounting Policies - Additional Information (Detail) $ / shares in Units, Bcf in Billions | 1 Months Ended | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Jan. 28, 2016 $ / shares | Dec. 31, 2015 USD ($) $ / shares Bcf | Sep. 30, 2015 $ / shares | Jun. 30, 2015 $ / shares | Mar. 31, 2015 $ / shares | Dec. 31, 2014 USD ($) $ / shares Bcf | Sep. 30, 2014 $ / shares | Jun. 30, 2014 $ / shares | Mar. 31, 2014 $ / shares | Dec. 31, 2015 USD ($) Entity Subsidiary mi $ / shares Bcf | Dec. 31, 2014 USD ($) $ / shares Bcf | Dec. 31, 2013 $ / shares
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Number of Subsidiaries | Entity |  |  |  |  |  |  |  |  |  | 3 |  |
Length Of Pipeline | mi |  |  |  |  |  |  |  |  |  | 86 |  |
Depreciation rate based on average depreciable property balance |  |  |  |  |  |  |  |  |  | 3.57% | 3.56% | 3.59%
Common stock dividend paid per share | $ / shares |  | $ 0.350 | $ 0.350 | $ 0.350 | $ 0.350 | $ 0.345 | $ 0.345 | $ 0.345 | $ 0.345 | $ 1.40 | $ 1.38 | $ 1.38
Common stock dividend per share, declared | $ / shares |  |  |  |  |  |  |  |  |  | $ 1.40 | $ 1.38 | $ 1.38
Average interest rates |  |  |  |  |  |  |  |  |  | 2.32% | 1.56% | 1.92%
Cost of removal obligation |  | $ 70,100,000 |  |  |  | $ 63,800,000 |  |  |  | $ 70,100,000 | $ 63,800,000 |
Regulatory Assets |  | $ 126,400,000 |  |  |  | $ 145,400,000 |  |  |  | $ 126,400,000 | $ 145,400,000 |
Number of Natural Gas Storage Outstanding | Bcf |  | 2.5 |  |  |  | 2.4 |  |  |  | 2.5 | 2.4 |
Investments in trading securities |  | $ 700,000 |  |  |  | $ 0 |  |  |  | $ 700,000 | $ 0 |
Intangible Assets, Purchase Adjustments |  |  |  |  |  |  |  |  |  | $ 7,200,000 |  |
Plant |  |  |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Intangible Asset Amortization Period |  |  |  |  |  |  |  |  |  | 10 years |  |
Bargain Purchase Adjustment |  |  |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Intangible Asset Amortization Period |  |  |  |  |  |  |  |  |  | 3 years |  |
Annual Electric Sales Volume |  |  |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Percentage of total sales volumes revenue subject to RDM |  |  |  |  |  |  |  |  |  | 27.00% |  |
Annual Natural Gas Sales Volume |  |  |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Percentage of total sales volumes revenue subject to RDM |  |  |  |  |  |  |  |  |  | 11.00% |  |
Maximum |  |  |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Cash equivalents maturity period |  |  |  |  |  |  |  |  |  | 3 months |  |
ISO-NE Obligations |  |  |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Cash Deposits |  | 2,300,000 |  |  |  | 6,300,000 |  |  |  | $ 2,300,000 | 6,300,000 |
Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Common stock dividend per share, declared | $ / shares | $ 1.42 |  |  |  |  |  |  |  |  |  |  |
Natural Gas Hedging Program |  |  |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Cash Deposits |  | 0 |  |  |  | 0 |  |  |  | $ 0 | $ 0 |
Quarterly Dividends |  |  |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Common stock dividend paid per share | $ / shares |  |  |  |  |  |  |  |  |  | $ 0.350 | $ 0.345 | $ 0.345
Quarterly Dividends | Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Common stock dividend per share, declared | $ / shares | 0.3550 |  |  |  |  |  |  |  |  |  |  |
Increase in dividend declared amount per share | $ / shares | $ 0.005 |  |  |  |  |  |  |  |  |  |  |
Deferred Storm Charges |  |  |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets |  | 15,400,000 |  |  |  | $ 21,200,000 |  |  |  | $ 15,400,000 | $ 21,200,000 |
Deferred Storm Charges | Recovered over the next two and a half years |  |  |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets |  | 6,600,000 |  |  |  |  |  |  |  | 6,600,000 |  |
Rate Case Costs and Other Expenditures | Recovered over the next seven years |  |  |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets |  | $ 7,400,000 |  |  |  |  |  |  |  | $ 7,400,000 |  |
Utilities |  |  |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Number of Subsidiaries | Subsidiary |  |  |  |  |  |  |  |  |  | 3 |  |
Unitil Service; Unitil Realty; and Unitil Resources |  |  |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Number of Subsidiaries | Subsidiary |  |  |  |  |  |  |  |  |  | 3 |  |

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