Retirement Benefits - Narrative - Plan Descriptions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Total assets	$ 957.0	$ 932.5
First 3% of Compensation Contributed
Defined Benefit Plan Disclosure [Line Items]
Percent of company matching compensation contributed	100.00%
Percent of participant's compensation contributed	3.00%
Cost of 401(k) plan	$ 21.8	21.4	$ 22.2
Defined Contribution 401(k) Plan, Next 3% Contributed
Defined Benefit Plan Disclosure [Line Items]
Percent of company matching compensation contributed	50.00%
Percent of participant's compensation contributed	3.00%
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Total assets	$ 957.0	932.5	894.8
Projected benefit obligations, total	1,381.5	1,349.8	$ 1,288.7
Unfunded pension obligation	424.5	417.3
Employer contributions	15.6	$ 1.4
Pension Benefits | Qualified Plan
Defined Benefit Plan Disclosure [Line Items]
Expected cash contributions	94.0
Employer contributions	14.1
Amount expected to be recoverable	$ 28.0