Pension Benefits - Changes in Fair Value of Plan Assets and Changes in Accumulated Benefit Obligations (Details) - Pension Plans, Defined Benefit - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Sep. 30, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plans’ Assets at Beginning of Year		$ 315.8	$ 391.7
Actual Return on Plan Assets		7.1	(65.1)
Benefits Paid	$ (90.0)	(100.9)	(13.7)
Settlement Benefits		(205.7)
Benefits Paid		(100.9)	(13.7)
Fair Value of Plan Assets at End of Year		16.3	315.8	$ 391.7
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Accumulated Postretirement Benefit Obligation at Beginning of Year		292.2	378.8
Interest Cost		8.4	8.7	7.2
Benefits Paid		(100.9)	(13.7)
Settlement Benefits		(205.7)	2.9
Actuarial Gains		6.0	(84.5)
Projected Benefit Obligation at End of Year		0.0	292.2	$ 378.8
Funded Status—Plan Assets in Excess of Projected Benefit Obligation		16.3	23.6
Unamortized Amount Reported in AOCI at End of Year		0.0	(63.1)
Accumulated Benefit Obligation at End of Year		$ 0.0	$ 289.3