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Nature of Business and Basis of Presentation - Additional Information (Detail) (USD $)
 6 Months Ended 1 Months Ended 1 Months Ended
Jun. 30, 2012
Jun. 30, 2011
Dec. 31, 2011
Dec. 31, 2010
Jan. 31, 2012
Finance
3.5% exchangeable senior notes due 2032
Jan. 09, 2012
Finance
3.5% exchangeable senior notes due 2032
Jul. 31, 2012
Finance
3.5% exchangeable senior notes due 2032
Interest Expense
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]              
Cash and cash equivalents $ 250,255,000 $ 131,446,000 $ 116,602,000 $ 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       144,316,000    
Interest payment on notes               $ 2,700,000