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Pension plans - Additional Information (Details) - USD ($)
$ in Millions
 1 Months Ended 12 Months Ended
Dec. 31, 2019
Jun. 30, 2019
Oct. 31, 2018
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]            
Net benefit obligations $ (35.2)     $ (35.2) $ (40.0)  
Defined benefit pension charge (credit)       (2.8) (4.0) $ (3.4)
Actuarial loss recognized due to curtailment event       1.8 0.0 0.0
Defined benefit pension (credit) charge       1.3 4.7 4.2
Defined benefit plan, expected amortization, next fiscal year 2.4     2.4    
Defined benefit plan, expected amortization of gain (loss), next fiscal year 2.8     2.8    
Defined benefit plan, expected amortization of prior service credit 0.4     0.4    
Curtailment gain   $ 1.8        
Immediate recognition of unamortized actuarial losses 0.8 $ 0.3        
Defined benefit plan, lump sum payment 2.7     2.7 0.0  
Decrease in defined benefit plan obligation 3.6          
Allowance included in benefit obligation       0.0 2.2  
Expected future employer contributions, next fiscal year 7.9     7.9    
Employer contribution amount       7.9    
U.K.            
Defined Benefit Plan Disclosure [Line Items]            
Net benefit obligations (30.5)     (30.5) (31.8)  
Defined benefit pension charge (credit)       (2.1) (4.1) (3.9)
Actuarial loss recognized due to curtailment event       0.0 0.0 0.0
Defined benefit plan, lump sum payment       0.0 0.0  
Allowance included in benefit obligation     $ 2.2 0.0 2.2  
Foreign Plan            
Defined Benefit Plan Disclosure [Line Items]            
Net benefit obligations $ (4.7)     (4.7) (8.2)  
Defined benefit pension charge (credit)       (0.7) 0.1 0.5
Actuarial loss recognized due to curtailment event       1.8 0.0 0.0
Defined benefit plan, lump sum payment       2.7 0.0  
Allowance included in benefit obligation       0.0 0.0  
Selling, General and Administrative Expenses            
Defined Benefit Plan Disclosure [Line Items]            
Defined benefit pension charge (credit)       $ 0.3 $ 0.7 $ 0.8