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Shareholders' equity (Details) - USD ($)
 3 Months Ended 9 Months Ended 50 Months Ended
Sep. 30, 2015
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Dec. 31, 2014
Aug. 01, 2011
Class of Stock [Line Items]            
Common stock, shares authorized (in shares) 195,000,000 195,000,000   195,000,000 195,000,000  
Stock repurchase program, authorized amount $ 130,000,000 $ 130,000,000   $ 130,000,000   $ 60,000,000
Stock repurchase program, additional authorized amount $ 70,000,000 $ 70,000,000   $ 70,000,000    
Repurchase of common stock (in shares) 0 2,400,000   7,186,521    
Repurchase of common stock, aggregate purchase price   $ 36,352,000   $ 86,276,000    
Stock repurchase program, remaining authorized repurchase amount $ 43,724,000 $ 43,724,000   $ 43,724,000    
Increase (Decrease) in Stockholders' Equity [Roll Forward]            
Repurchase of common stock (in shares) 0 2,400,000   7,186,521    
Series A Convertible Preferred Stock [Member]            
Class of Stock [Line Items]            
Repurchase of common stock (in shares)   0        
Increase (Decrease) in Stockholders' Equity [Roll Forward]            
Shares, Beginning balance (in shares)   78,141 79,620      
Repurchase of common stock (in shares)   0        
Conversion of convertible preferred stock (in shares)   (1,146) (950)      
Issuance for stock compensation plans (in shares)   0 0      
Shares, Ending balance (in shares) 76,995 76,995 78,670 76,995    
Treasury Stock [Member]            
Class of Stock [Line Items]            
Repurchase of common stock (in shares)   2,400,000        
Increase (Decrease) in Stockholders' Equity [Roll Forward]            
Shares, Beginning balance (in shares)   4,786,521 4,786,521      
Repurchase of common stock (in shares)   2,400,000        
Conversion of convertible preferred stock (in shares)   0 0      
Issuance for stock compensation plans (in shares)   0 0      
Shares, Ending balance (in shares) 7,186,521 7,186,521 4,786,521 7,186,521    
Common Stock [Member]            
Class of Stock [Line Items]            
Repurchase of common stock (in shares)   (2,400,000)        
Increase (Decrease) in Stockholders' Equity [Roll Forward]            
Shares, Beginning balance (in shares)   89,064,582 88,710,277      
Repurchase of common stock (in shares)   (2,400,000)        
Conversion of convertible preferred stock (in shares)   114,524 95,039      
Issuance for stock compensation plans (in shares)   152,597 132,554      
Shares, Ending balance (in shares) 86,931,703 86,931,703 88,937,870 86,931,703