Employee Benefit Plans and Other Postretirement Benefits - Components of Net Periodic Benefit Cost and Other Comprehensive (Income) Loss (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Interest Cost, Statement of Income or Comprehensive Income [Extensible Enumeration]		Interest Income (Expense), Net	Interest Income (Expense), Net	Interest Income (Expense), Net
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost		$ 14.1	$ 19.3	$ 21.4
Interest cost		55.8	34.7	29.6
Expected return on plan assets		57.1	(55.7)	(63.1)
Net amortization of unrecognized amounts, Prior service cost (credit)		5.2	3.6	3.8
Net amortization of unrecognized amounts, Actuarial loss (gain)		4.1	3.4	10.4
Net periodic benefit cost		22.1	5.3	2.1
Changes in plan assets and benefit obligations recognized in other comprehensive (income) loss:
Actuarial net loss (gain)		35.5	28.6	(82.7)
Prior service cost (credit)		2.9	15.1	2.1
Amortization of prior service cost (credit)		(5.2)	(3.6)	(3.8)
Amortization of actuarial loss (gain)		(4.1)	(3.4)	(10.4)
Total recognized in other comprehensive loss (income)	[1]	41.9	36.7	(94.8)
Total recognized in net periodic benefit cost and other comprehensive loss (income) (pre-tax)		19.8	42.0	(92.7)
Postretirement Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost		0.2	0.2	0.2
Interest cost		0.5	0.3	0.3
Expected return on plan assets		0.0	0.0
Net amortization of unrecognized amounts, Prior service cost (credit)		(0.4)	(0.4)	(0.4)
Net amortization of unrecognized amounts, Actuarial loss (gain)		(0.4)	(0.4)	(0.5)
Net periodic benefit cost		(0.1)	(0.3)	(0.4)
Changes in plan assets and benefit obligations recognized in other comprehensive (income) loss:
Actuarial net loss (gain)		(1.5)	(3.4)	(1.3)
Prior service cost (credit)		0.0	0.0	(0.1)
Amortization of prior service cost (credit)		0.4	0.4	0.4
Amortization of actuarial loss (gain)		0.4	0.4	0.5
Total recognized in other comprehensive loss (income)	[1]	(2.3)	(2.6)	2.1
Total recognized in net periodic benefit cost and other comprehensive loss (income) (pre-tax)		$ (2.2)	$ (2.9)	$ 1.7

[1] Accumulated losses in excess of 10% of the greater of the projected benefit obligation or the market-related value of assets will be recognized on a straight-line basis over the average remaining service period of active employees in PCA plans (which is between six and nine years) and over the average remaining lifetime of inactive participants in the Boise plan (which is approximately 23 years), to the extent that losses are not offset by gains in subsequent years.