Subsequent events (Details) (USD $)
In Millions, except Share data, unless otherwise specified
1 Months Ended 2 Months Ended 1 Months Ended
Jul. 31, 2013
Feb. 29, 2012
Dec. 31, 2014
Jan. 31, 2015
Subsequent events        
Number of shares of common stock issued in initial public offering 4,255,000 6,325,000 1,346,676  
Net proceeds from shares of common stock issued in initial public offering   $ 56.8    
Subsequent events        
Subsequent events        
Number of shares of common stock issued in initial public offering       276,663
Net proceeds from shares of common stock issued in initial public offering       2.5
Subsequent events | Underwriting agreement        
Subsequent events        
Shares offering through underwriting agreement       1,087,500
Number of shares of common stock issued in initial public offering       8,337,500
Share price for underwriting agreement       $ 6.50
Net proceeds from shares of common stock issued in initial public offering       $ 50.9