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Shareholders' equity (Details) - USD ($)
 8 Months Ended 12 Months Ended 60 Months Ended
Dec. 31, 2015
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2015
Mar. 31, 2015
Dec. 31, 2011
Dec. 31, 2008
Class of Stock [Line Items]                
Common stock, shares authorized (shares) 195,000,000 195,000,000 195,000,000   195,000,000      
Common stock, par value (in dollars per share) $ 0.01 $ 0.01 $ 0.01   $ 0.01      
Common stock, shares issued 94,224,571 94,224,571 93,851,103   94,224,571      
Common stock, shares outstanding 87,038,050 87,038,050 89,064,582   87,038,050      
Preferred stock, shares authorized (shares) 5,000,000 5,000,000 5,000,000   5,000,000      
Preferred stock, shares issued (shares)               160,000
Preferred stock, par value (in dollars per share) $ 0.01 $ 0.01 $ 0.01   $ 0.01      
Liquidation preference (in dollars per share) $ 0.01 $ 0.01     $ 0.01      
Conversion of convertible preferred stock (shares) 100 100     100      
Authorized repurchase amount $ 130,000,000 $ 130,000,000     $ 130,000,000   $ 60,000,000  
Additional repurchase amount           $ 70,000,000    
Treasury stock acquired, 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 (shares) 0       7,186,521      
Series A Convertible Preferred Stock [Member]                
Class of Stock [Line Items]                
Preferred stock, shares outstanding 76,539 76,539 78,141   76,539      
Preferred stock, par value (in dollars per share) $ 0.01 $ 0.01     $ 0.01      
Increase (Decrease) in Stockholders' Equity [Roll Forward]                
Shares, Beginning balance   78,141 79,620 80,283        
Repurchase of common stock (shares)   0            
Conversion of convertible preferred stock (shares)   (1,602) (1,479) (663)        
Issuance for share-based compensation plans (shares)   0 0 0        
Shares, Ending balance 76,539 76,539 78,141 79,620 76,539      
Treasury [Member]                
Increase (Decrease) in Stockholders' Equity [Roll Forward]                
Shares, Beginning balance   4,786,521 4,786,521 4,786,521        
Repurchase of common stock (shares)   2,400,000            
Conversion of convertible preferred stock (shares)   0 0 0        
Issuance for share-based compensation plans (shares)   0 0 0        
Shares, Ending balance 7,186,521 7,186,521 4,786,521 4,786,521 7,186,521      
Common stock [Member]                
Increase (Decrease) in Stockholders' Equity [Roll Forward]                
Shares, Beginning balance   89,064,582 88,710,277 88,548,637        
Repurchase of common stock (shares)   (2,400,000)            
Conversion of convertible preferred stock (shares)   160,162 147,952 66,244        
Issuance for share-based compensation plans (shares)   213,306 206,353 95,396        
Shares, Ending balance 87,038,050 87,038,050 89,064,582 88,710,277 87,038,050