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Share repurchases (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
 1 Months Ended 9 Months Ended
Nov. 07, 2017
Sep. 30, 2017
Oct. 10, 2017
Equity, Class of Treasury Stock [Line Items]      
Repurchase of common stock (in shares)   5,557  
Value of treasury stock acquired   $ 348,801  
Average cost of treasury stock (usd per share)   $ 62.77  
Prior shares under repurchase authorization (in shares)   247,039  
Subsequent Event      
Equity, Class of Treasury Stock [Line Items]      
Repurchase of common stock (in shares) 5,889    
Value of treasury stock acquired $ 352,873    
Average cost of treasury stock (usd per share) $ 59.92    
Stock repurchase additional authorization amount     $ 1,252,961
Remaining repurchase authorized amount $ 1,228,391