Description of Business and Basis of Presentation - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011
End of Period
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Amount excluded from the cash and cash equivalents balance during the period	$ 426.6	$ 286.4
Beginning of Period
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Amount excluded from the cash and cash equivalents balance during the period	$ 310.5	$ 256.7