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Shareholders' equity (Details) - USD ($)
 3 Months Ended 36 Months Ended 87 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Mar. 31, 2018
Mar. 31, 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      
Common stock, shares issued (in shares) 94,762,390   94,762,390 94,762,390 94,731,298      
Common stock, shares, outstanding (in shares) 87,575,869   87,575,869 87,575,869 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 $ 70,000,000 $ 60,000,000  
Treasury stock, shares, acquired (in shares)     0 7,186,521        
Treasury stock, value, acquired, cost method       $ 86,276,000        
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)   87,262,115            
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) 74,231   74,231 74,231 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) (133) (48)            
Issuance for stock compensation plans (in shares) 0 0            
Shares, Ending balance (in shares) 74,231.000 75,577 74,231.000 74,231.000        
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) 13,343 4,795            
Issuance for stock compensation plans (in shares) 17,749 6,423            
Shares, Ending balance (in shares) 87,575,869.000   87,575,869.000 87,575,869.000