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Pension and Other Post-Retirement Benefits - Fair Value Pension Plan Assets (Details) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Fair value of pension plan assets    
Assets $ 465.5 $ 432.7
Short-term investment funds [Member]    
Fair value of pension plan assets    
Assets 4.6 5.9
Common stock collective funds [Member]    
Fair value of pension plan assets    
Assets 223.0 181.3
Fixed income collective trust funds [Member]    
Fair value of pension plan assets    
Assets 226.0 232.4
Real estate investment funds [Member]    
Fair value of pension plan assets    
Assets $ 11.9 $ 13.1