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Marketable securities - Summary of Available for Sale Securities Held (Detail) - USD ($)
$ in Thousands
Jun. 30, 2015
Dec. 31, 2014
Schedule Of Available For Sale Securities [Line Items]    
Amortized Cost $ 343,199 $ 144,229
Unrealized Gains 37 6
Unrealized Losses (90) (77)
Fair Value 343,146 144,158
U.S. Government Agency Securities [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Amortized Cost 329,359 131,589
Unrealized Gains 29 6
Unrealized Losses (89) (59)
Fair Value 329,299 131,536
Certificates of Deposit [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Amortized Cost 13,840 12,640
Unrealized Gains 8  
Unrealized Losses (1) (18)
Fair Value $ 13,847 $ 12,622