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Nature of operations (Details) (USD $)
In Thousands, except Share data, unless otherwise specified
 0 Months Ended 12 Months Ended
Apr. 17, 2012
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Aug. 02, 2010
private_equity_funds
Equity [Line Items]          
Number of private equity funds (in private equity funds)         3
Common stock share price (in dollars per share) $ 20.00        
Proceeds from issuance of initial public offering $ 256,400 $ 0 $ 256,381 $ 0  
Proceeds from concurrent private placement 50,000 0 50,000 0  
IPO [Member]
          
Equity [Line Items]          
Stock issued during the period (in shares) 13,889,470        
Private Placement [Member]
          
Equity [Line Items]          
Stock issued during the period (in shares) 2,666,666        
Stockholder [Member]
          
Equity [Line Items]          
Proceeds from issuance of initial public offering $ 147,200        
Stockholder [Member] | IPO [Member]
          
Equity [Line Items]          
Stock issued during the period (in shares) 7,900,000        
Underwriters With An Option To Purchase Shares [Member] | IPO [Member]
          
Equity [Line Items]          
Stock issued during the period (in shares) 2,842,104