Stockholders' Equity - Reclassifications Out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Income before taxes	[1]	$ 245.3	$ 208.6	$ 431.8	$ 387.7
Income tax benefit		(58.7)	(65.4)	(105.1)	(127.1)
Net income		186.6	143.2	326.7	260.6
Unfunded Employee Benefit Obligations | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Amortization of prior service costs		(1.7)	(1.4)	(3.3)	(2.9)
Amortization of actuarial gains / (losses)		(2.3)	(1.9)	(4.6)	(3.7)
Income before taxes		(4.0)	(3.3)	(7.9)	(6.6)
Income tax benefit		1.0	1.2	1.9	2.3
Net income		(3.0)	(2.1)	(6.0)	(4.3)
Treasury Lock | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Interest expense, net		(1.3)	(1.4)	(2.7)	(2.8)
Income tax benefit		0.3	0.6	0.7	1.2
Net income		$ (1.0)	$ (0.8)	$ (2.0)	$ (1.6)

[1] (a)Effective January 1, 2018, the Company adopted ASU 2017-07, Compensation: Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost and applied this standard retrospectively to the prior period reflected herein. This new standard requires the presentation of non-service cost components of net periodic benefits expense to be shown separately outside the subtotal of operating income in the income statement. See Note 2, New and Recently Adopted Accounting Standards, for more information. The components of our financial statements affected by the change in presentation of operating and non-operating pension expense as originally reported in 2017 and as adjusted for the requirements per the new standard are as follows (dollars in millions): Segment income (loss) Three Months Ended June 30, 2017 As Reported Non-Operating Pension Adjustment Three Months Ended June 30, 2017 Adjusted Packaging $224.5 $1.7 $226.2 Paper 29.1 (1.9) 27.2 Corporate (19.8) 0.5 (19.3) Income from operations 233.8 0.3 234.1 Interest expense, net and other (25.2) (0.3) (25.5) Income before taxes $208.6 $— $208.6 Segment income (loss) Six Months Ended June 30, 2017 As Reported Non-Operating Pension Adjustment Six Months Ended June 30, 2017 Adjusted Packaging $415.3 $3.3 $418.6 Paper 58.9 (3.8) 55.1 Corporate (37.3) 1.1 (36.2) Income from operations 436.9 0.6 437.5 Interest expense, net and other (49.2) (0.6) (49.8) Income before taxes $387.7 $— $387.7