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Shareholders' equity (Details) - USD ($)
 9 Months Ended 30 Months Ended 81 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 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) 94,513,473   94,513,473 94,513,473 94,437,418      
Common stock, shares outstanding (in shares) 87,326,952 87,075,587 87,326,952 87,326,952 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) 75,299   75,299 75,299        
Increase (Decrease) in Stockholders' Equity [Roll Forward]                
Shares, Beginning balance (in shares) 75,625 76,539            
Conversion of convertible preferred stock (in shares) (326) (161)            
Issuance for stock compensation plans (in shares) 0 0            
Shares, Ending balance (in shares) 75,299 76,378 75,299 75,299        
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)   87,038,050            
Conversion of convertible preferred stock (in shares) 32,619 16,109            
Issuance for stock compensation plans (in shares) 43,436 21,428            
Shares, Ending balance (in shares) 87,326,952   87,326,952 87,326,952