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Subsequent events (Details) - USD ($)
Feb. 28, 2019
Jan. 31, 2019
Dec. 31, 2018
Oct. 31, 2018
Apr. 30, 2018
Feb. 28, 2018
Oct. 31, 2017
Jul. 31, 2017
May 31, 2017
Feb. 28, 2017
Subsequent Event [Line Items]                    
Share repurchase program, authorized amount     $ 100,000,000 $ 200,000,000 $ 200,000,000 $ 200,000,000 $ 200,000,000 $ 250,000,000 $ 300,000,000 $ 350,000,000
Subsequent Event                    
Subsequent Event [Line Items]                    
Share repurchase program, authorized amount $ 200,000,000                  
Subsequent Event | Unsecured | Senior Notes, 4.45% Due 2021                    
Subsequent Event [Line Items]                    
Face amount   $ 700,000,000                
Stated interest rate   4.45%                
Subsequent Event | Unsecured | Senior Notes, 4.875% Due 2024                    
Subsequent Event [Line Items]                    
Face amount   $ 400,000,000                
Stated interest rate   4.875%