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Shareholders' equity (Details) - USD ($)
$ in Thousands
 6 Months Ended 47 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Mar. 26, 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  
Stock repurchase program, authorized amount $ 130,000   $ 130,000     $ 60,000
Stock repurchase program, additional authorized amount       $ 70,000    
Repurchase of common stock (in shares) 2,400,000   7,186,521      
Repurchase of common stock, aggregate purchase price $ 36,352   $ 86,276      
Stock repurchase program, remaining authorized repurchase amount $ 43,724   $ 43,724      
Increase (Decrease) in Stockholders' Equity [Roll Forward]            
Repurchase of common stock (in shares) 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) (1,014) (394)        
Issuance for stock compensation plans (in shares) 0 0        
Shares, Ending balance (in shares) 77,127 79,226 77,127      
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) 7,186,521.000 4,786,521 7,186,521.000      
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) 101,327 39,455        
Issuance for stock compensation plans (in shares) 135,043 55,029        
Shares, Ending balance (in shares) 86,900,952.000 88,804,761 86,900,952.000