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Summary of Significant Accounting Policies - Additional Information (Detail) Bcf in Billions |  | 3 Months Ended | 6 Months Ended |
--- | --- | --- | --- | ---
Jul. 25, 2018 USD ($) | Jun. 30, 2018 USD ($) Bcf | Jun. 30, 2017 USD ($) Bcf | Jun. 30, 2018 USD ($) Subsidiary mi Bcf | Jun. 30, 2017 USD ($) Bcf | Dec. 31, 2017 USD ($) Bcf
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Length Of Pipeline | mi |  |  |  | 86 |  |
Revenue sharing payments |  | $ 200,000 | $ 300,000 | $ 500,000 | $ 500,000 |
Other |  | 83,400,000 | 79,400,000 | 227,900,000 | 203,700,000 |
Operation and Maintenance Expense |  | 17,800,000 | 16,500,000 | 35,100,000 | 32,500,000 |
Cost of removal obligation |  | 87,800,000 | 82,300,000 | 87,800,000 | 82,300,000 | $ 84,300,000
Regulatory Assets |  | $ 131,900,000 | $ 132,000,000 | $ 131,900,000 | $ 132,000,000 | $ 149,100,000
Number of Natural Gas Storage Outstanding | Bcf |  | 0.0 | 1.2 | 0.0 | 1.2 | 0.6
Investments in trading securities |  | $ 5,200,000 | $ 3,400,000 | $ 5,200,000 | $ 3,400,000 | $ 3,600,000
Subsequent Event | Second Amended Credit Facility [Member] |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Borrowing limit | $ 120,000,000 |  |  |  |  |
Increase in borrowing limit | $ 50,000,000 |  |  |  |  |
ASU 2014-09 |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Revenue sharing payments |  |  | 300,000 |  | 500,000 |
Operation and Maintenance Expense |  |  | 16,200,000 |  | 32,000,000 |
Non Service |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Non-service cost net periodic benefit costs |  | 1,300,000 | 1,100,000 | 2,900,000 | 2,700,000 |
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 |  |  |  |
Maximum | Subsequent Event | Second Amended Credit Facility [Member] |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Applicable margin rate | 1.25% |  |  |  |  |
Minimum | Subsequent Event | Second Amended Credit Facility [Member] |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Applicable margin rate | 1.125% |  |  |  |  |
ISO-NE Obligations |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Cash Deposits |  | $ 2,200,000 | 2,000,000 | $ 2,200,000 | 2,000,000 | 2,900,000
Natural Gas Hedging Program |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Cash Deposits |  | 0 | 0 | 0 | 0 | 0
Deferred Storm Charges |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Regulatory Assets |  | 6,800,000 | 7,700,000 | 6,800,000 | 7,700,000 | $ 7,200,000
Deferred Storm Charges | Recovered over Next Year |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Regulatory Assets |  | 100,000 |  | 100,000 |  |
Environmental and Rate Case Costs and Other Expenditures | Recovered over the next seven years |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Regulatory Assets |  | $ 6,300,000 |  | $ 6,300,000 |  |
Restatement Adjustment [Member] | ASU 2014-09 |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Other |  |  | (300,000) |  | (500,000) |
Operation and Maintenance Expense |  |  | $ (300,000) |  | $ (500,000) |
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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