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Defined benefit and 401(k) savings plan - Components of Net Periodic Pension Cost (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Retirement Benefits [Abstract]    
Service cost $ 0.7 $ 1.9
Interest cost 0.3 0.1
Expected return on plan assets (0.4) (0.8)
Amortization of loss 0.0 0.1
Amortization of prior service credit 0.0 (0.1)
Settlements 0.0 (0.4)
Net periodic benefit cost $ 0.6 $ 0.8