Stockholders' Equity - Reclassifications Out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions		3 Months Ended															12 Months Ended
		Dec. 31, 2019	[4]	Sep. 30, 2019	[5]	Jun. 30, 2019	Mar. 31, 2019	[6]	Dec. 31, 2018	[7]	Sep. 30, 2018	[8]	Jun. 30, 2018	[9]	Mar. 31, 2018	[10]	Dec. 31, 2019		Dec. 31, 2018	Dec. 31, 2017
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Interest expense, net																	$ (128.8)	[1]	$ (95.1)	$ (102.6)	[2],[3]
Income before taxes																	917.0		970.5	828.6	[3]
Income tax benefit																	(220.6)		(232.5)	(160.0)
Net income		$ 136.2		$ 179.8		$ 193.6	$ 186.8		$ 204.6		$ 206.7		$ 186.6		$ 140.1		696.4		738.0	$ 668.6
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	[11]																(6.0)		(6.6)
Amortization of actuarial gains / (losses)	[11]																(6.7)		(9.2)
Income before taxes																	(12.7)		(15.8)
Income tax benefit																	3.2		4.0
Net income																	(9.5)		(11.8)
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	[12]																(18.1)		(5.3)
Income tax benefit																	7.9		1.3
Net income																	$ (10.2)		$ (4.0)

[1] Includes $38.7 million of charges related to the Company’s November 2019 debt refinancing, which included redemption premiums and the write-offs of remaining balances of treasury locks and unamortized debt issuance costs.

[2] Includes $1.8 million of expense related to the write-off of deferred debt issuance costs in connection with the December 2017 debt refinancing.

[3] Includes “Additions to property, plant, and equipment” and excludes cash used for “Acquisitions of businesses, net of cash acquired” as reported on our Consolidated Statements of Cash Flows.

[4] Includes $0.4 million of charges consisting of closure costs related related to the second quarter 2018 discontinuation of uncoated free sheet and coated one-side white paper grades at the Wallula Washington mill associated with the conversion of the No. 3 paper machine to produce virgin kraft linerboard ($0.3 million after-tax or $0.00 per diluted share) and $38.7 million of charges related to the Company’s November 2019 debt refinancing, which included redemption premiums and the write-offs of remaining balances of treasury locks and unamortized debt issuance costs as well as $3.2 million of income tax benefit from the stranded tax effects in Accumulated Other Comprehensive Income related to the write-offs of the treasury locks ($25.9 million after-tax or $0.28 per diluted share). Also includes $0.3 million of charges consisting of closure costs related to corrugated products facilities, partially offset by income from the sale of a building related to a closed corrugated products facility ($0.2 million after-tax or $0.00 per diluted share).

[5] Includes $3.0 million of charges for the disposal of fixed assets related to the containerboard mill conversion at our DeRidder, Louisiana mill ($2.3 million after-tax or $0.02 per diluted share).

[6] Includes $0.6 million of charges consisting of closure costs related to the second quarter 2018 discontinuation of uncoated free sheet and coated one-side white paper grades at the Wallula Washington mill associated with the conversion of the No. 3 paper machine to produce virgin kraft linerboard ($0.5 million after-tax or $0.01 per diluted share).

[7] Includes $3.6 million of charges related to the second quarter 2018 discontinuation of uncoated free sheet and coated one-side white paper grades at the Wallula, Washington mill associated with the conversion of the No. 3 paper machine to produce virgin kraft linerboard ($2.7 million after-tax or $0.03 per diluted share) and $0.1 million of charges for acquisition and integration costs related to recent acquisitions ($0.1 million after-tax or $0.00 per diluted share). Also includes $2.0 million of income tax benefit for the re-measurement of our net deferred tax liability to our 2017 measurement period adjustments in accordance with SEC Staff Accounting Bulletin No. 118 (SAB 118), Income Tax Accounting Implications of the Tax Cuts and Jobs Act ($0.02 per diluted share).

[8] Includes $4.0 million of charges related to the second quarter 2018 discontinuation of uncoated free sheet and coated one-side white paper grades at the Wallula Washington mill associated with the conversion of the No. 3 paper machine to produce virgin kraft linerboard ($2.9 million after-tax or $0.04 per diluted share) and $1.3 million of charges consisting of closure costs related to corrugated products facilities and a corporate administration facility ($1.0 million after-tax or $0.01 per diluted share). Also includes $0.5 million of costs for the property damage insurance deductible for a weather-related incident at one of the corrugated products facilities ($0.4 million after-tax or $0.00 per diluted share) and $0.1 million of charges for acquisition and integration costs related to recent acquisitions ($0.01 million after-tax or $0.00 per diluted share).

[9] Includes $0.2 million of charges consisting of closure costs related to corrugated products facilities and a corporate administration facility ($0.2 million after-tax and $0.00 per diluted share) and $13.6 million of charges related to the second quarter 2018 discontinuation of uncoated free sheet and coated one-side white paper grades at the Wallula Washington mill associated with the conversion of the No. 3 paper machine to produce virgin kraft linerboard ($10.2 million after-tax or $0.11 per diluted share).

[10] Includes $0.3 million of charges consisting of closure costs related to corrugated products facilities and a corporate administration facility ($0.2 million after-tax or $0.00 per diluted share) and $8.8 million of charges related to the second quarter 2018 discontinuation of uncoated free sheet and coated one-side white paper grades at the Wallula Washington mill associated with the conversion of the No. 3 paper machine to produce virgin kraft linerboard ($6.6 million after-tax or $0.07 per diluted share).

[11] These AOCI components are included in the computation of net pension and postretirement benefit costs. See Note 13, Employee Benefit Plans and Other Postretirement Benefits, for additional information.

[12] This AOCI component is included in interest expense, net. The remaining balances of the treasury locks were written off  as a result of the Company’s November 2019 debt refinancing. For a discussion of treasury lock derivative instrument activity, see Note 16, Derivative Instruments and Hedging Activities, for additional information.