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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

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.

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