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Shareholders' equity - Stock Repurchase Program (Details) - USD ($)
$ / shares in Units, $ in Millions
1 Months Ended 3 Months Ended 12 Months Ended
Dec. 31, 2017
Jun. 30, 2017
Dec. 31, 2018
Dec. 31, 2018
Dec. 01, 2018
Dec. 01, 2017
Aug. 31, 2017
Aug. 31, 2016
Feb. 29, 2016
2016 Stock Repurchase Program                  
Accelerated Share Repurchases [Line Items]                  
Common stock repurchase, authorized amount             $ 125.0 $ 125.0 $ 50.0
June 2017 ASR Program [Member]                  
Accelerated Share Repurchases [Line Items]                  
Common stock repurchase, authorized amount   $ 20.0 $ 60.0 $ 60.0          
Repurchase of common stock (in shares)   460,887   1,138,387          
Stock repurchase, average cost per share (in USD per share)   $ 43.39              
December 2017 ASR Program [Member]                  
Accelerated Share Repurchases [Line Items]                  
Common stock repurchase, authorized amount     $ 234.6 $ 234.6   $ 50.0      
Repurchase of common stock (in shares) 677,500   182,171            
Settlement of accelerated share repurchase program           $ 40.0      
Stock repurchase, average cost per share (in USD per share) $ 59.04                
2017 Stock Repurchase Program                  
Accelerated Share Repurchases [Line Items]                  
Common stock repurchase, authorized amount             $ 275.0    
Repurchase of common stock (in shares)       1,772,658          
Stock repurchase, average cost per share (in USD per share)     $ 62.69            
2018 Stock Repurchase Program [Member] [Member]                  
Accelerated Share Repurchases [Line Items]                  
Common stock repurchase, authorized amount         $ 100.0