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Summary of Significant Accounting Policies - Additional Information (Detail) Bcf in Billions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2018 USD ($) Subsidiary mi Bcf | Mar. 31, 2017 USD ($) Bcf | Dec. 31, 2017 USD ($) Bcf
Significant Accounting Policies [Line Items] |  |  |
Length Of Pipeline | mi | 86 |  |
Revenue sharing payments | $ 300,000 | $ 300,000 |
Other | 1,300,000 | 1,700,000 |
Operation and Maintenance Expense | 17,300,000 | 16,000,000 |
Cost of removal obligation | 86,600,000 | 79,700,000 | $ 84,300,000
Regulatory Assets | $ 145,800,000 | $ 135,500,000 | $ 149,100,000
Number of Natural Gas Storage Outstanding | Bcf | 0.0 | 1.2 | 0.6
Investments in trading securities | $ 5,100,000 | $ 3,300,000 | $ 3,600,000
ASU 2014-09 |  |  |
Significant Accounting Policies [Line Items] |  |  |
Revenue sharing payments |  | 300,000 |
Operation and Maintenance Expense |  | 15,700,000 |
Non Service |  |  |
Significant Accounting Policies [Line Items] |  |  |
Non-service cost net periodic benefit costs | 1,500,000 | 1,600,000 |
Not Designated as Hedging Instruments |  |  |
Significant Accounting Policies [Line Items] |  |  |
Derivative Assets, not designated as hedging instruments | $ 0 | 300,000 | 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 | $ 3,300,000 | 2,600,000 | 2,900,000
Natural Gas Hedging Program |  |  |
Significant Accounting Policies [Line Items] |  |  |
Cash Deposits | 0 | 0 | 0
Restatement Adjustment [Member] | ASU 2014-09 |  |  |
Significant Accounting Policies [Line Items] |  |  |
Other |  | (300,000) |
Operation and Maintenance Expense |  | (300,000) |
Deferred Storm Charges |  |  |
Significant Accounting Policies [Line Items] |  |  |
Regulatory Assets | 8,000,000 | $ 8,600,000 | $ 7,200,000
Deferred Storm Charges | Recovered over Next Year |  |  |
Significant Accounting Policies [Line Items] |  |  |
Regulatory Assets | 500,000 |  |
Environmental and Rate Case Costs and Other Expenditures | Recovered over the next seven years |  |  |
Significant Accounting Policies [Line Items] |  |  |
Regulatory Assets | $ 6,500,000 |  |
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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