Pension Benefits - Changes in Fair Value of Plan Assets and Changes in Accumulated Benefit Obligations (Details) - USD ($) $ in Millions			12 Months Ended
	Aug. 22, 2019	Jul. 13, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Employer Contributions	$ 55.3	$ 5.1
Pension Plans, Defined Benefit
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plans’ Assets at Beginning of Year			$ 664.6	$ 525.3
Actual Return on Plan Assets			92.1	113.2
Employer Contributions			0.0	55.3
Benefits Paid			(145.9)	(29.2)
Defined Benefit Plan, Plan Assets, Payment for Settlement			(205.4)	0.0
Fair Value of Plan Assets at End of Year			405.4	664.6	$ 525.3
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Accumulated Postretirement Benefit Obligation at Beginning of Year			660.5	580.5
Interest Cost			16.5	22.3	20.3
Benefits Paid			(145.9)	(29.2)
Settlement Benefits			(205.4)	0.0
Actuarial (Gains) Losses			56.6	86.9
Projected Benefit Obligation at End of Year			382.3	660.5	$ 580.5
Funded Status—Plan Assets in Excess (Deficit) of Projected Benefit Obligation			23.1	4.1
Unamortized Amount Reported in AOCI at End of Year			(68.2)	(145.7)
Accumulated Benefit Obligation at End of Year			$ 382.3	$ 660.4