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Nature of Business and Basis of Presentation - Additional Information (Detail) (USD $)
 9 Months Ended 1 Months Ended
Sep. 30, 2012
Dec. 31, 2011
Sep. 30, 2011
Dec. 31, 2010
Jul. 31, 2012
Finance
3.5% exchangeable senior notes due 2032
Jan. 31, 2012
Finance
3.5% exchangeable senior notes due 2032
Jan. 09, 2012
Finance
3.5% exchangeable senior notes due 2032
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]              
Cash and cash equivalents $ 215,110,000 $ 116,602,000 $ 125,855,000 $ 31,442,000      
Convertible senior notes             150,000,000
Convertible senior notes, stated interest rate             3.50%
Convertible senior notes, maturity date           Jan. 15, 2032  
Proceeds from private placement of convertible senior notes 144,316,000         144,300,000  
Interest payment on notes $ 2,713,000       $ 2,700,000