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Retirement Plans - Components of Net Periodic Benefit Cost (Credit) (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Defined Benefit Pension Plan        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 5.5 $ 5.1 $ 11.1 $ 10.2
Interest cost 1.6 2.3 3.1 4.6
Expected return on assets (3.0) (3.2) (6.1) (6.6)
Settlement loss 10.2
Other 1.3 0.7 2.7 1.6
Total benefit cost (credit) 5.4 4.9 10.8 20.0
Retiree Health Care Plan        
Defined Benefit Plan Disclosure [Line Items]        
Interest cost 0.1 0.1 0.2
Prior service credit (0.2) (0.2) (0.4) (0.4)
Other 0.1 0.1
Total benefit cost (credit) $ (0.1) $ (0.1) $ (0.2) $ (0.2)