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Retirement Plans - Components of Net Periodic Benefit Cost (Credit) (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Defined Benefit Pension Plan    
Defined Benefit Plan Disclosure [Line Items]    
Service cost $ 5.6 $ 5.1
Interest cost 1.5 2.3
Expected return on assets (3.1) (3.4)
Settlement loss   10.2
Other 1.4 0.9
Total benefit cost (credit) 5.4 15.1
Retiree Health Care Plan    
Defined Benefit Plan Disclosure [Line Items]    
Interest cost 0.1 0.1
Settlement loss 0.0 0.0
Other (0.2) (0.2)
Total benefit cost (credit) $ (0.1) $ (0.1)