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Summary of Significant Accounting Policies - Additional Information (Detail) Bcf in Billions |  |  | 3 Months Ended
--- | --- | --- | ---
Dec. 31, 2015 USD ($) Bcf | Mar. 31, 2015 USD ($) Bcf | Mar. 31, 2016 USD ($) Subsidiary mi Bcf
Significant Accounting Policies [Line Items] |  |  |
Length Of Pipeline | mi |  |  | 86
Cost of Removal Obligations | $ 70,100,000 | $ 65,700,000 | $ 72,300,000
Regulatory Assets | $ 126,400,000 | $ 139,900,000 | $ 131,900,000
Number of Natural Gas Storage Outstanding | Bcf | 2.5 | 2.0 | 1.8
Investments in trading securities | $ 700,000 | $ 0 | $ 700,000
Reduction of other assets | (2,900,000) | (3,100,000) |
Reduction of long-term debt | (2,900,000) | (3,100,000) |
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 | 1,500,000 | $ 2,100,000
Natural Gas Hedging Program |  |  |
Significant Accounting Policies [Line Items] |  |  |
Cash Deposits | 0 | 0 | 0
Not Designated as Hedging Instruments | Maximum |  |  |
Significant Accounting Policies [Line Items] |  |  |
Derivative Assets, not designated as hedging instruments |  | 100,000 | 100,000
Deferred Storm Charges |  |  |
Significant Accounting Policies [Line Items] |  |  |
Regulatory Assets | $ 15,400,000 | $ 20,100,000 | 13,800,000
Deferred Storm Charges | Recovered over the next two and a half years |  |  |
Significant Accounting Policies [Line Items] |  |  |
Regulatory Assets |  |  | 5,800,000
Environmental Costs Rate Case Costs And Other Expenditures | Recovered over the next seven and a half years |  |  |
Significant Accounting Policies [Line Items] |  |  |
Regulatory Assets |  |  | $ 8,300,000
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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