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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
--- | --- | --- | ---
Feb. 02, 2018 $ / shares | Dec. 31, 2017 USD ($) $ / shares Bcf | Sep. 30, 2017 $ / shares | Jun. 30, 2017 $ / shares | Mar. 31, 2017 $ / shares | Dec. 31, 2016 USD ($) $ / shares Bcf | Sep. 30, 2016 $ / shares | Jun. 30, 2016 $ / shares | Mar. 31, 2016 $ / shares | Dec. 31, 2017 USD ($) Entity Subsidiary mi $ / shares Bcf | Dec. 31, 2016 USD ($) $ / shares Bcf | Dec. 31, 2015 $ / shares
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Number of Subsidiaries | Entity |  |  |  |  |  |  |  |  |  | 3 |  |
Length Of Pipeline | mi |  |  |  |  |  |  |  |  |  | 86 |  |
Depreciation rate based on average depreciable property balance |  |  |  |  |  |  |  |  |  | 3.45% | 3.49% | 3.57%
Common stock dividend paid per share | $ / shares |  | $ 0.360 | $ 0.360 | $ 0.360 | $ 0.360 | $ 0.355 | $ 0.355 | $ 0.355 | $ 0.355 | $ 1.44 | $ 1.42 | $ 1.40
Common stock dividend per share, declared | $ / shares |  |  |  |  |  |  |  |  |  | $ 1.44 | $ 1.42 | $ 1.40
Average interest rates |  |  |  |  |  |  |  |  |  | 2.90% | 2.18% | 2.32%
Cost of removal obligation |  | $ 84,300,000 |  |  |  | $ 77,000,000 |  |  |  | $ 84,300,000 | $ 77,000,000 |
Regulatory Assets |  | $ 149,100,000 |  |  |  | $ 142,000,000 |  |  |  | $ 149,100,000 | $ 142,000,000 |
Number of Natural Gas Storage Outstanding | Bcf |  | 0.6 |  |  |  | 2.0 |  |  |  | 0.6 | 2.0 |
Investments in trading securities |  | $ 3,600,000 |  |  |  | $ 1,900,000 |  |  |  | $ 3,600,000 | $ 1,900,000 |
Intangible Assets, Purchase Adjustments |  |  |  |  |  |  |  |  |  | $ 2,300,000 |  |
Plant |  |  |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Intangible Asset Amortization Period |  |  |  |  |  |  |  |  |  | 10 years |  |
Bargain Purchase Adjustment |  |  |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Intangible Asset Amortization Period |  |  |  |  |  |  |  |  |  | 1 year |  |
Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Common stock dividend per share, declared | $ / shares | $ 1.46 |  |  |  |  |  |  |  |  |  |  |
Quarterly Dividends |  |  |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Common stock dividend paid per share | $ / shares |  |  |  |  |  |  |  |  |  | $ 0.360 | $ 0.355 | $ 0.350
Quarterly Dividends | Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Common stock dividend per share, declared | $ / shares | 0.365 |  |  |  |  |  |  |  |  |  |  |
Increase in dividend declared amount per share | $ / shares | $ 0.005 |  |  |  |  |  |  |  |  |  |  |
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 |  | 2,900,000 |  |  |  | 2,800,000 |  |  |  | $ 2,900,000 | $ 2,800,000 |
Natural Gas Hedging Program |  |  |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Cash Deposits |  | 0 |  |  |  | 0 |  |  |  | 0 | 0 |
Deferred Storm Charges |  |  |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets |  | 7,200,000 |  |  |  | $ 9,600,000 |  |  |  | 7,200,000 | $ 9,600,000 |
Deferred Storm Charges | Recovered over Next Year |  |  |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets |  | 900,000 |  |  |  |  |  |  |  | 900,000 |  |
Environmental and Rate Case Costs and Other Expenditures | Recovered over the next seven years |  |  |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets |  | $ 7,600,000 |  |  |  |  |  |  |  | $ 7,600,000 |  |
Unitil Service; Unitil Realty; and Unitil Resources |  |  |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Number of Subsidiaries | Subsidiary |  |  |  |  |  |  |  |  |  | 3 |  |
Utilities |  |  |  |  |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Number of Subsidiaries | Subsidiary |  |  |  |  |  |  |  |  |  | 3 |  |

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