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Summary of Significant Accounting Policies - Additional Information (Detail) Bcf in Billions | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2018 USD ($) Bcf | Sep. 30, 2017 USD ($) Bcf | Sep. 30, 2018 USD ($) Subsidiary mi Bcf | Sep. 30, 2017 USD ($) Bcf | Dec. 31, 2017 USD ($) Bcf
Significant Accounting Policies [Line Items] |  |  |  |  |
Length Of Pipeline | mi |  |  | 86 |  |
Revenue sharing payments | $ 300,000 | $ 300,000 | $ 800,000 | $ 800,000 |
Other | 87,100,000 | 82,600,000 | 315,000,000 | 286,300,000 |
Operation and Maintenance Expense | 16,400,000 | 16,900,000 | 51,500,000 | 49,400,000 |
Cost of removal obligation | 91,200,000 | 84,800,000 | 91,200,000 | 84,800,000 | $ 84,300,000
Regulatory Assets | $ 138,000,000 | $ 133,300,000 | $ 138,000,000 | $ 133,300,000 | $ 149,100,000
Number of Natural Gas Storage Outstanding | Bcf | 0.0 | 1.2 | 0.0 | 1.2 | 0.6
Investments in trading securities | $ 5,300,000 | $ 3,400,000 | $ 5,300,000 | $ 3,400,000 | $ 3,600,000
ASU 2014-09 |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Revenue sharing payments |  | 300,000 |  | 800,000 |
Operation and Maintenance Expense |  | 16,600,000 |  | 48,600,000 |
Non Service |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Non-service cost net periodic benefit costs | 1,200,000 | 900,000 | 4,100,000 | 3,600,000 |
Not Designated as Hedging Instruments |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Derivative Assets, not designated as hedging instruments | $ 0 | 100,000 | $ 0 | 100,000 | 100,000
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 | $ 3,200,000 | 4,800,000 | $ 3,200,000 | 4,800,000 | 2,900,000
Natural Gas Hedging Program |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Cash Deposits | 0 | 0 | 0 | 0 | $ 0
Environmental and Rate Case Costs and Other Expenditures | Recovered over the next seven years |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Regulatory Assets | $ 6,100,000 |  | $ 6,100,000 |  |
Restatement Adjustment [Member] | ASU 2014-09 |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |
Other |  | (300,000) |  | (800,000) |
Operation and Maintenance Expense |  | $ (300,000) |  | $ (800,000) |
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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