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Significant Accounting Policies - Additional Information (Details) - USD ($)
$ in Millions
1 Months Ended 9 Months Ended 18 Months Ended
Jun. 30, 2015
May. 31, 2014
Sep. 30, 2015
Mar. 31, 2013
Dec. 31, 2014
Schedule Of Accounting Policies [Line Items]          
Carrying value of cash held in money market funds     $ 52.5   $ 55.0
Expected term (years)     6 years 3 months    
Pfizer License Agreement [Member]          
Schedule Of Accounting Policies [Line Items]          
Revenue $ 20.0 $ 15.0      
Pfizer License Agreement [Member] | Up-Front Payment Arrangement [Member]          
Schedule Of Accounting Policies [Line Items]          
Amount received and recognized as revenue       $ 22.5  
Joint steering committees period       1 year 6 months  
Maximum [Member]          
Schedule Of Accounting Policies [Line Items]          
Share-based compensation arrangement by share-based payment award, option grant term     10 years