Summary of Significant Accounting Policies (Details) (USD $)
0 Months Ended 12 Months Ended
Oct. 07, 2014
Jun. 25, 2014
Jun. 11, 2014
Apr. 24, 2014
Dec. 31, 2014
item
Dec. 31, 2013
Oct. 07, 2014
Jun. 11, 2014
Summary of Basis of Presentation and Significant Accounting Policies                
Conversion of convertible preferred stock into common stock (in shares)     19,465,132          
Number of shares of common stock issued in satisfaction of accumulated dividends accrued on the preferred stock     2,862,654          
Initial public offering                
Shares sold 2,750,000   6,500,000          
Share issue price (in dollars per share) $ 18.25   $ 8.00       $ 18.25 $ 8.00
Proceeds from the sale of common stock $ 46,900,000       $ 103,804,000      
Options exercised by underwriters (in shares) 378,524 512,744            
Aggregate proceeds received, net of underwriting discounts and commissions and offering costs 53,400,000 50,400,000            
Common Stock, Shares Authorized     200,000,000   200,000,000 100,000,000   200,000,000
Reverse stock-split ratio       2.28        
Number of operating segment         1      
Number of geographic segment         1      
Marketable Securities                
Other-than-temporary impairment losses         0      
Realized gains or losses on the sale of securities         0 0    
Impairment of Long-Lived Assets                
Impairment charges         $ 0      
Stock-Based Compensation                
Expected dividend yield (as a percent)         0.00%      
Common Stock                
Summary of Basis of Presentation and Significant Accounting Policies                
Conversion of convertible preferred stock into common stock (in shares)         22,327,786      
Initial public offering                
Shares sold         10,141,268      
Warrants issued for purchase of shares of common stock (in shares)     149,452         149,452