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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
Dec. 31, 2021
Retirement Benefits [Abstract]      
Service cost $ 0.7 $ 1.9 $ 2.4
Interest cost 0.3 0.1 0.0
Expected return on plan assets (0.4) (0.8) (0.8)
Amortization of loss 0.0 0.1 0.6
Amortization of prior service credit 0.0 (0.1) (0.2)
Settlements 0.0 (0.4) 0.0
Net periodic benefit cost $ 0.6 $ 0.8 $ 2.0