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Pension and Other Postretirement Benefits - Net Periodic Benefit Cost (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Pension      
Defined Benefit Plan Disclosure [Line Items]      
Service cost $ 4.9 $ 4.0 $ 4.3
Interest cost 11.4 13.3 12.4
Expected return on plan assets (20.8) (18.3) (21.1)
Amortization of prior service costs 0.1 0.1 0.1
Amortization of net loss 6.5 6.6 5.2
Net periodic benefit cost 2.1 5.7 0.9
OPEB      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 0.2 0.2 0.2
Interest cost 3.1 4.5 4.2
Expected return on plan assets 0.0 0.0 0.0
Amortization of prior service costs (3.2) (4.9) (7.3)
Amortization of net loss 2.2 2.3 4.0
Net periodic benefit cost $ 2.3 $ 2.1 $ 1.2