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Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Summary Of Significant Accounting Policies [Line Items]      
Research and development costs $ 16.1 $ 29.8 $ 20.3
Expected lives of rated securities, minimum 2 years    
Expected lives of rated securities, maximum 52 years    
Deferred Revenue related to rated securities 82 79 76
Account receivable related to commercial paper $ 22 $ 24 $ 25
Concentration risk, percentage 10.00% 10.00%