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Fair Value Measurements (Details) - USD ($)
$ in Thousands
Jun. 30, 2015
Dec. 31, 2014
Fair value measurements    
Assets transferred between the fair value measurement classifications $ 0  
Liabilities transferred between the fair value measurement classifications 0  
Fair Value Measurements | Fair value    
Fair value measurements    
Money market funds and cash equivalents 68,641 $ 18,677
Marketable securities 252,408 173,023
Total financial assets at fair value 321,049 191,700
Fair Value Measurements | Level 2    
Fair value measurements    
Money market funds and cash equivalents 68,641 18,677
Marketable securities 252,408 173,023
Total financial assets at fair value $ 321,049 $ 191,700