Employee Benefit Plans and Other Postretirement Benefits - Change in Benefit Obligations and Change in Fair Value of Plan Assets Related to Pension and Postretirement Plans (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at fair value at beginning of period		$ 1,100.0
Company contributions		50.0	$ 50.0	$ 50.0
Fair value of plan assets at plan year end		267.2	1,100.0
Amounts Recognized on Consolidated Balance Sheets:
Noncurrent liabilities		(106.4)	(141.8)
Pension Plan [Member]
Change in Benefit Obligations [Roll Forward]
Benefit obligation at beginning of period		1,167.1	1,503.8
Service cost		14.1	19.3	21.4
Interest cost		55.8	34.7	29.6
Plan amendments		2.9	15.1
Actuarial loss (gain) (a)	[1]	(23.3)	(348.0)
Participant contributions		0.0	0.0
Benefits paid		(61.1)	(57.8)
Benefit obligation at plan year end		1,202.1	1,167.1	1,503.8
Accumulated benefit obligation portion of above		1,173.3	1,131.0
Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at fair value at beginning of period		1,055.3	1,382.7
Actual return on plan assets		115.9	(320.9)
Company contributions		51.5	51.3
Participant contributions		0.0	0.0
Benefits paid		(61.1)	(57.8)
Fair value of plan assets at plan year end		1,161.6	1,055.3	1,382.7
Underfunded status		(40.5)	(111.8)
Amounts Recognized on Consolidated Balance Sheets:
Noncurrent assets		26.5	0.0
Current liabilities		(1.7)	(1.9)
Noncurrent liabilities		(65.3)	(109.9)
Accrued obligation recognized at December 31		(40.5)	(111.8)
Amounts Recognized in Accumulated Other Comprehensive Loss (Income) (Pre-Tax):
Prior service cost (credit)		29.2	31.5
Actuarial loss (gain)		(116.0)	155.6
Total		145.2	187.1
Postretirement Plans
Change in Benefit Obligations [Roll Forward]
Benefit obligation at beginning of period		9.5	13.6
Service cost		0.2	0.2	0.2
Interest cost		0.5	0.3	0.3
Plan amendments		(0.0)	0.0
Actuarial loss (gain) (a)	[1]	(1.5)	(3.4)
Participant contributions		0.4	0.5
Benefits paid		(1.6)	(1.7)
Benefit obligation at plan year end		10.5	9.5	13.6
Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at fair value at beginning of period		0.0	0.0
Actual return on plan assets		0.0	0.0
Company contributions		1.2	1.2
Participant contributions		0.4	0.5
Benefits paid		(1.6)	(1.7)
Fair value of plan assets at plan year end		0.0	0.0	$ 0.0
Underfunded status		(10.5)	(9.5)
Amounts Recognized on Consolidated Balance Sheets:
Noncurrent assets		0.0	0.0
Current liabilities		(0.5)	(0.5)
Noncurrent liabilities		(10.0)	(9.0)
Accrued obligation recognized at December 31		(10.5)	(9.5)
Amounts Recognized in Accumulated Other Comprehensive Loss (Income) (Pre-Tax):
Prior service cost (credit)		(3.8)	(4.2)
Actuarial loss (gain)		4.7	(6.7)
Total		$ (8.5)	$ (10.9)

[1] For the year ended December 31, 2023, the most significant driver of the increase in aggregate benefit obligations for the pension and OPEB plans was the actuarial losses due to a decrease in the discount rate assumption, PCA Hourly Pension Plan benefit multiplier increases, and changes in salary scale and medical claims assumptions to better reflect anticipated future experience. For the year ended December 31, 2022, the most significant driver of the decrease in aggregate benefit obligations for the pension and OPEB plans was the actuarial gains due to an increase in the discount rate assumption.