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Components of Accrued Revenue (Detail) - USD ($) $ in Millions | Sep. 30, 2018 | Dec. 31, 2017 | Sep. 30, 2017
--- | --- | --- | ---
Deferred Revenue Arrangement [Line Items] |  |  |
Accrued Revenue | $ 36.0 | $ 53.3 | $ 39.2
Unbilled Revenues |  |  |
Deferred Revenue Arrangement [Line Items] |  |  |
Accrued Revenue | 9.6 | 13.8 | 7.9
Regulatory Assets |  |  |
Deferred Revenue Arrangement [Line Items] |  |  |
Accrued Revenue | $ 26.4 | $ 39.5 | $ 31.3

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- Definition Unbilled amounts due for services rendered or to be rendered, actions taken or to be taken, or a promise to refrain from taking certain actions in accordance with the terms of a legally binding agreement between the entity and, at a minimum, one other party. An example would be amounts associated with contracts or programs where the recognized revenue for performance thereunder exceeds the amounts billed under the terms thereof as of the date of the balance sheet. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 912 -SubTopic 310 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=68054760&loc=d3e55302-109406 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.3(c)(3)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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