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Shareholders' equity (Details) - USD ($)
 6 Months Ended 24 Months Ended 75 Months Ended
Jun. 30, 2017
Jun. 30, 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      
Common stock, par value (in dollars per share) $ 0.01       $ 0.01      
Common stock, shares issued (in shares) 94,502,486       94,437,418      
Common stock, shares outstanding (in shares) 87,315,965       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              
Preferred stock, shares authorized (in shares) 5,000,000       5,000,000      
Preferred stock par value (in dollars per share) $ 0.01       $ 0.01      
Preferred shares issued (in shares)               160,000
Number of shares issued for each share of preferred stock (in shares) 100              
Series A Convertible Preferred Stock [Member]                
Class of Stock [Line Items]                
Preferred stock par value (in dollars per share) $ 0.01              
Preferred shares outstanding (in shares) 75,346       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) (279) (161)            
Issuance for stock compensation plans (in shares) 0 0            
Shares, Ending balance (in shares) 75,346 76,378            
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            
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) 27,904 16,109            
Issuance for stock compensation plans (in shares) 37,164 21,428            
Shares, Ending balance (in shares) 87,315,965 87,075,587