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Shareholders' equity (Details) - USD ($)
 9 Months Ended 18 Months Ended 60 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Dec. 31, 2015
Dec. 31, 2011
Dec. 31, 2008
Class of Stock [Line Items]            
Common stock, shares authorized (in shares) 195,000,000   195,000,000 195,000,000    
Common stock, par value (in dollars per share) $ 0.01   $ 0.01 $ 0.01    
Common stock, shares issued (in shares) 94,262,108   94,262,108 94,224,571    
Common stock, shares outstanding (in shares) 87,075,587   87,075,587 87,038,050    
Stock repurchase program, authorized amount       $ 130,000,000 $ 60,000,000  
Repurchase of common stock (in shares)     0 7,186,521    
Repurchase of common stock, aggregate purchase price       $ 86,276,000    
Stock repurchase program, remaining authorized repurchase amount $ 43,724,000   $ 43,724,000      
Preferred stock, shares authorized (in shares) 5,000,000   5,000,000 5,000,000    
Preferred stock par value (in dollars per share) $ 0.01   $ 0.01 $ 0.01    
Preferred shares issued (in shares)           160,000
Number of shares issued for each share of preferred stock (in shares) 100   100      
Increase (Decrease) in Stockholders' Equity [Roll Forward]            
Repurchase of common stock (in shares)     0 7,186,521    
Series A Convertible Preferred Stock [Member]            
Class of Stock [Line Items]            
Repurchase of common stock (in shares)   0        
Preferred stock par value (in dollars per share) $ 0.01   $ 0.01      
Preferred shares outstanding (in shares) 76,378   76,378 76,539    
Increase (Decrease) in Stockholders' Equity [Roll Forward]            
Shares, Beginning balance (in shares) 76,539 78,141        
Repurchase of common stock (in shares)   0        
Conversion of convertible preferred stock (in shares) (161) (1,146)        
Issuance for stock compensation plans (in shares) 0 0        
Shares, Ending balance (in shares) 76,378 76,995 76,378 76,539    
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) 7,186,521 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 7,186,521 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) 87,038,050 89,064,582        
Repurchase of common stock (in shares)   (2,400,000)        
Conversion of convertible preferred stock (in shares) 16,109 114,524        
Issuance for stock compensation plans (in shares) 21,428 152,597        
Shares, Ending balance (in shares) 87,075,587 86,931,703 87,075,587 87,038,050