Retirement Benefits - Narrative - Summary of Company's Plan Assets, Benefit Obligations, and Funded Status (Details) - Pension Benefits - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Actuarial losses	$ 97.4	$ 116.9
Qualified Plan
Defined Benefit Plan Disclosure [Line Items]
Discount rate (percentage)	2.52%	3.28%	4.27%