Employee Benefit Plans and Other Postretirement Plans - Components of Net Periodic Benefit Cost and Other Comprehensive (Income) Loss (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Components of net periodic benefit cost
Net amortization of unrecognized amounts, Curtailment loss	$ 0		$ 10.9	[1]	$ 0
Estimated Net Periodic Benefit Cost and Amortization of Other Comprehensive Income for The Next Year
Estimated net periodic benefit cost for 2015	14.0
Pension Plans
Components of net periodic benefit cost
Service cost	22.7		24.5		22.4
Interest cost	45.9		21.5		14.8
Expected return on plan assets	(50.7)		(21.4)		(12.1)
Special termination benefits	0.3		0		0
Net amortization of unrecognized amounts, Prior service cost	6.5		6.2		6.0
Net amortization of unrecognized amounts, Actuarial loss	0.6		4.7		4.9
Net amortization of unrecognized amounts, Curtailment loss	0		10.9	[2]	0
Net periodic benefit cost	25.3		46.4		36.0
Changes in plan assets and benefit obligations recognized in other comprehensive (income) loss
Actuarial net (gain) loss	146.4		(58.7)		19.9
Prior service cost	2.6		13.8		2.3
Amortization of prior service cost	(6.5)		(6.2)		(6.0)
Amortization of actuarial loss	(0.6)		(4.7)		(4.9)
Curtailment loss	0		(10.9)	[2]	0
Total recognized in other comprehensive (income) loss (b)	141.9	[3]	(66.7)	[3]	11.3	[3]
Total recognized in net periodic benefit cost and other comprehensive (income) loss - pretax	167.2		(20.3)		47.3
Postretirement Plans
Components of net periodic benefit cost
Service cost	1.6		2.1		1.9
Interest cost	1.2		1.3		1.2
Expected return on plan assets	0		0		0
Special termination benefits	0		0		0
Net amortization of unrecognized amounts, Prior service cost	(0.2)		(0.4)		(0.4)
Net amortization of unrecognized amounts, Actuarial loss	0.1		0.5		0.5
Net amortization of unrecognized amounts, Curtailment loss	0		0		0
Net periodic benefit cost	2.7		3.5		3.2
Changes in plan assets and benefit obligations recognized in other comprehensive (income) loss
Actuarial net (gain) loss	4.2		(7.8)		1.7
Prior service cost	0		0		0
Amortization of prior service cost	0.2		0.4		0.4
Amortization of actuarial loss	(0.1)		(0.5)		(0.5)
Curtailment loss	0		0		0
Total recognized in other comprehensive (income) loss (b)	4.3	[3]	(7.9)	[3]	1.6	[3]
Total recognized in net periodic benefit cost and other comprehensive (income) loss - pretax	$ 7.0		$ (4.4)		$ 4.8

[1] Includes $10.9 million of non-cash pension curtailment charges related to pension plan changes in which certain hourly corrugated plant and containerboard mill employees will transition from a defined benefit pension plan to a defined contribution (401k) plan.

[2] We recognized curtailment losses in "Other expense, net" in the Consolidated Statements of Income for recent USW negotiations, resulting in the bifurcation of the active USW population between those grandfathered in the current formula (with continued accruals) and non-grandfathered in the current formula (frozen benefits at the contract date).

[3] 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, which is between seven to ten years, to the extent that losses are not offset by gains in subsequent years. The estimated net loss and prior service cost that will be amortized from "Accumulated other comprehensive loss" into pension expense in 2015 is $14.0 million.