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Components of Accrued Revenue (Detail) (USD $) In Millions, unless otherwise specified | Mar. 31, 2013 | Dec. 31, 2012 | Mar. 31, 2012
--- | --- | --- | ---
Deferred Revenue Arrangement [Line Items] |  |  |
Accrued Revenue | $ 37.7 | $ 58.1 | $ 40.9
Regulatory Asset |  |  |
Deferred Revenue Arrangement [Line Items] |  |  |
Accrued Revenue | 27.3 | 46.6 | 32.3
Unbilled Revenues |  |  |
Deferred Revenue Arrangement [Line Items] |  |  |
Accrued Revenue | $ 10.4 | $ 11.5 | $ 8.6

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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://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Research Bulletin (ARB) -Number 43 -Section A -Paragraph 4, 21 -Chapter 11 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -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 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 3 -Subparagraph c(3) -Article 5 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 912 -SubTopic 310 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6471842&loc=d3e55302-109406 + Details
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