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Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions, Bcf in Billions |  |  | 9 Months Ended
--- | --- | --- | ---
Dec. 31, 2015 USD ($) Bcf | Sep. 30, 2015 USD ($) Bcf | Sep. 30, 2016 USD ($) Subsidiary mi Bcf
Significant Accounting Policies [Line Items] |  |  |
Length Of Pipeline | mi |  |  | 86
Cost of Removal Obligations | $ 70.1 | $ 69.9 | $ 77.4
Regulatory Assets | $ 126.4 | $ 125.4 | $ 124.5
Number of Natural Gas Storage Outstanding | Bcf | 2.5 | 3.7 | 2.9
Investments in trading securities | $ 0.7 | $ 0.7 | $ 1.9
Reduction of other assets | (2.9) | (3.0) |
Reduction of long-term debt | (2.9) | (3.0) |
Maximum |  |  |
Significant Accounting Policies [Line Items] |  |  |
Cash equivalents maturity period |  |  | 3 months
ISO-NE Obligations |  |  |
Significant Accounting Policies [Line Items] |  |  |
Cash Deposits | 2.3 | 2.9 | $ 3.5
Not Designated as Hedging Instruments |  |  |
Significant Accounting Policies [Line Items] |  |  |
Derivative Assets, not designated as hedging instruments |  | 0.1 | 0.2
Not Designated as Hedging Instruments | Maximum |  |  |
Significant Accounting Policies [Line Items] |  |  |
Derivative Assets, not designated as hedging instruments | 0.1 |  |
Deferred Storm Charges |  |  |
Significant Accounting Policies [Line Items] |  |  |
Regulatory Assets | $ 15.4 | $ 16.9 | 11.0
Deferred Storm Charges | Recovered over the next two and a half years |  |  |
Significant Accounting Policies [Line Items] |  |  |
Regulatory Assets |  |  | 4.1
Environmental Costs Rate Case Costs And Other Expenditures | Recovered over the next seven and a half years |  |  |
Significant Accounting Policies [Line Items] |  |  |
Regulatory Assets |  |  | $ 8.4
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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