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Shareholders' equity (Details) - USD ($)
 3 Months Ended 12 Months Ended 20 Months Ended 24 Months Ended 60 Months Ended 75 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2016
Mar. 31, 2017
Dec. 31, 2015
Mar. 31, 2017
Jun. 30, 2016
Mar. 31, 2015
Dec. 31, 2011
Dec. 31, 2008
Class of Stock [Line Items]                          
Common stock, shares authorized (shares) 195,000,000   195,000,000 195,000,000   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 $ 0.01 $ 0.01 $ 0.01        
Common stock, shares issued (shares) 94,448,636   94,437,418 94,224,571   94,437,418 94,448,636 94,224,571 94,448,636        
Common stock, shares outstanding (shares) 87,262,115   87,250,897 87,038,050   87,250,897 87,262,115 87,038,050 87,262,115        
Preferred stock, shares authorized (shares) 5,000,000   5,000,000 5,000,000   5,000,000 5,000,000 5,000,000 5,000,000        
Preferred stock, shares issued (shares)                         160,000
Preferred stock, par value (in dollars per share) $ 0.01   $ 0.01 $ 0.01   $ 0.01 $ 0.01 $ 0.01 $ 0.01        
Liquidation preference (in dollars per share)     $ 0.01     $ 0.01              
Conversion of convertible preferred stock (shares) 100   100     100 100   100 100      
Authorized repurchase amount     $ 130,000,000 $ 130,000,000   $ 130,000,000   $ 130,000,000       $ 60,000,000  
Additional repurchase amount                     $ 70,000,000    
Treasury stock acquired, Aggregate purchase price       $ 36,352,000       $ 86,276,000 $ 86,276,000        
Stock repurchase program, Remaining authorized repurchase amount $ 43,724,000   $ 43,724,000     $ 43,724,000 $ 43,724,000   $ 43,724,000        
Increase (Decrease) in Stockholders' Equity [Roll Forward]                          
Repurchase of common stock (shares)           0 0 7,186,521 7,186,521        
Series A Convertible Preferred Stock [Member]                          
Class of Stock [Line Items]                          
Preferred stock, shares outstanding (shares) 75,577   75,625 76,539   75,625 75,577 76,539 75,577        
Preferred stock, par value (in dollars per share) $ 0.01   $ 0.01     $ 0.01 $ 0.01   $ 0.01        
Increase (Decrease) in Stockholders' Equity [Roll Forward]                          
Beginning balance (shares) 75,625 76,539 76,539 78,141 79,620                
Repurchase of common stock (shares)       0                  
Conversion of convertible preferred stock (shares) (48) (93) (914) (1,602) (1,479)                
Issuance for share-based compensation plans (shares) 0 0 0 0 0                
Ending balance (shares) 75,577 76,446 75,625 76,539 78,141 75,625 75,577 76,539 75,577        
Treasury [Member]                          
Increase (Decrease) in Stockholders' Equity [Roll Forward]                          
Beginning balance (shares) 7,186,521 7,186,521 7,186,521 4,786,521 4,786,521                
Repurchase of common stock (shares)       2,400,000                  
Conversion of convertible preferred stock (shares) 0 0 0 0 0                
Issuance for share-based compensation plans (shares) 0 0 0 0 0                
Ending balance (shares) 7,186,521 7,186,521 7,186,521 7,186,521 4,786,521 7,186,521 7,186,521 7,186,521 7,186,521        
Common stock [Member]                          
Increase (Decrease) in Stockholders' Equity [Roll Forward]                          
Beginning balance (shares) 87,250,897 87,038,050 87,038,050 89,064,582 88,710,277                
Repurchase of common stock (shares)       (2,400,000)                  
Conversion of convertible preferred stock (shares) 4,795 9,305 91,362 160,162 147,952                
Issuance for share-based compensation plans (shares) 6,423 12,378 121,485 213,306 206,353                
Ending balance (shares) 87,262,115 87,059,733 87,250,897 87,038,050 89,064,582 87,250,897 87,262,115 87,038,050 87,262,115