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Retirement Benefits - Summary of Actual Return and Rate of Return on Assets (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Retirement Benefits [Abstract]      
Actual gain (loss) on assets $ 117.1 $ 144.8 $ (57.8)
Actual rate of return (loss) on assets 15.60% 18.00% (5.20%)