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Shareholders' equity (Details) - USD ($)
 3 Months Ended 12 Months Ended 60 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Mar. 31, 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 $ 0.001    
Common stock, shares issued 94,246,254   94,246,254 94,224,571    
Common stock, shares outstanding 87,059,733   87,059,733 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 5,000,000   5,000,000 5,000,000    
Preferred stock par value (in dollars per share) $ 0.01   $ 0.01 $ 0.001    
Preferred shares issued           160,000
Number of shares issued for each share of preferred stock 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 0        
Preferred stock par value (in dollars per share) $ 0.01   $ 0.01      
Preferred shares outstanding 76,446   76,446 76,539    
Increase (Decrease) in Stockholders' Equity [Roll Forward]            
Shares, Beginning balance (in shares) 76,539 78,141 78,061      
Repurchase of common stock (in shares) 0 0        
Conversion of convertible preferred stock (in shares) (93) (80)        
Issuance for stock compensation plans (in shares) 0 0        
Shares, Ending balance (in shares) 76,446 78,061 76,446 76,539    
Treasury Stock [Member]            
Class of Stock [Line Items]            
Repurchase of common stock (in shares) 0 1,200,000        
Increase (Decrease) in Stockholders' Equity [Roll Forward]            
Shares, Beginning balance (in shares) 7,186,521 4,786,521 5,986,521      
Repurchase of common stock (in shares) 0 1,200,000        
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 5,986,521 7,186,521 7,186,521    
Common Stock [Member]            
Class of Stock [Line Items]            
Repurchase of common stock (in shares) 0 (1,200,000)        
Increase (Decrease) in Stockholders' Equity [Roll Forward]            
Shares, Beginning balance (in shares) 87,038,050 89,064,582 87,883,357      
Repurchase of common stock (in shares) 0 (1,200,000)        
Conversion of convertible preferred stock (in shares) 9,305 7,947        
Issuance for stock compensation plans (in shares) 12,378 10,828        
Shares, Ending balance (in shares) 87,059,733 87,883,357 87,059,733 87,038,050