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Components of Accrued Revenue (Detail) - USD ($) $ in Millions | Dec. 31, 2015 | Dec. 31, 2014
--- | --- | ---
Deferred Revenue Arrangement [Line Items] |  |
Accrued Revenue | $ 38.4 | $ 48.5
Regulatory Assets |  |
Deferred Revenue Arrangement [Line Items] |  |
Accrued Revenue | 26.8 | 37.8
Unbilled Revenues |  |
Deferred Revenue Arrangement [Line Items] |  |
Accrued Revenue | $ 11.6 | $ 10.7

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