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Shareholders' equity (Details) - USD ($)
 3 Months Ended 24 Months Ended 75 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Mar. 31, 2017
Mar. 31, 2017
Dec. 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   195,000,000 195,000,000 195,000,000      
Common stock, par value (in dollars per share) $ 0.01   $ 0.01 $ 0.01 $ 0.01      
Common stock, shares issued (in shares) 94,448,636   94,448,636 94,448,636 94,437,418      
Common stock, shares outstanding (in shares) 87,262,115   87,262,115 87,262,115 87,250,897      
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 $ 43,724,000        
Preferred stock, shares authorized (in shares) 5,000,000   5,000,000 5,000,000 5,000,000      
Preferred stock par value (in dollars per share) $ 0.01   $ 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   100 100        
Series A Convertible Preferred Stock [Member]                
Class of Stock [Line Items]                
Preferred stock par value (in dollars per share) $ 0.01   $ 0.01 $ 0.01        
Preferred shares outstanding (in shares) 75,577   75,577 75,577 75,625      
Increase (Decrease) in Stockholders' Equity [Roll Forward]                
Shares, Beginning balance (in shares) 75,625 76,539            
Conversion of convertible preferred stock (in shares) (48) (93)            
Issuance for stock compensation plans (in shares) 0 0            
Shares, Ending balance (in shares) 75,577 76,446 75,577 75,577        
Treasury Stock [Member]                
Increase (Decrease) in Stockholders' Equity [Roll Forward]                
Shares, Beginning balance (in shares) 7,186,521 7,186,521            
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]                
Increase (Decrease) in Stockholders' Equity [Roll Forward]                
Shares, Beginning balance (in shares) 87,250,897 87,038,050            
Conversion of convertible preferred stock (in shares) 4,795 9,305            
Issuance for stock compensation plans (in shares) 6,423 12,378            
Shares, Ending balance (in shares) 87,262,115 87,059,733 87,262,115 87,262,115