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Shareholders' equity (Details) - USD ($)
 6 Months Ended 39 Months Ended 90 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2018
Dec. 31, 2017
Mar. 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) 94,787,612   94,787,612 94,787,612 94,731,298      
Common stock, shares, outstanding (in shares) 87,601,091   87,601,091 87,601,091 87,544,777      
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        
Stock repurchase program, authorized amount $ 130,000,000   $ 130,000,000 $ 130,000,000   $ 70,000,000 $ 60,000,000  
Treasury stock, shares, acquired (in shares)     0 7,186,521        
Treasury stock, value, acquired, cost method       $ 86,300,000        
Stock repurchase program, remaining authorized repurchase amount $ 43,700,000   $ 43,700,000 $ 43,700,000        
Increase (Decrease) in Stockholders' Equity [Roll Forward]                
Shares, Ending balance (in shares)   87,315,965            
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 stock, shares outstanding (in shares) 74,123   74,123 74,123 74,364,000      
Increase (Decrease) in Stockholders' Equity [Roll Forward]                
Shares, Beginning balance (in shares) 74,364 75,625            
Conversion of convertible preferred stock (in shares) (241) (279)            
Issuance for stock compensation plans (in shares) 0 0            
Shares, Ending balance (in shares) 74,123.000 75,346 74,123.000 74,123.000        
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) 7,186,521.000 7,186,521 7,186,521.000 7,186,521.000        
Common Stock [Member]                
Increase (Decrease) in Stockholders' Equity [Roll Forward]                
Shares, Beginning balance (in shares) 87,544,777 87,250,897            
Conversion of convertible preferred stock (in shares) 24,167 27,904            
Issuance for stock compensation plans (in shares) 32,147 37,164            
Shares, Ending balance (in shares) 87,601,091.000   87,601,091.000 87,601,091.000