Earnings Per Share - Computation of Basic and Diluted Income Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended															12 Months Ended
	Dec. 31, 2020	[1]	Sep. 30, 2020	[2]	Jun. 30, 2020	[3]	Mar. 31, 2020	[4]	Dec. 31, 2019	[5]	Sep. 30, 2019	[6]	Jun. 30, 2019	Mar. 31, 2019	[7]	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Numerator:
Net income	$ 123.5		$ 139.1		$ 56.7		$ 141.7		$ 136.2		$ 179.8		$ 193.6	$ 186.8		$ 461.0	$ 696.4	$ 738.0
Less: distributed and undistributed earnings allocated to participating securities																(3.6)	(5.2)	(5.7)
Net income attributable to common stockholders																$ 457.4	$ 691.2	$ 732.3
Denominator:
Weighted average common shares outstanding (in shares)																94.1	93.8	93.7
Effect of dilutive securities (in shares)																0.3	0.3	0.2
Diluted common shares outstanding (in shares)																94.4	94.1	93.9
Basic income per common share (in dollars per share)	$ 1.30		$ 1.47		$ 0.60		$ 1.50		$ 1.44		$ 1.90		$ 2.05	$ 1.98		$ 4.86	$ 7.36	$ 7.82
Diluted income per common share (in dollars per share)	$ 1.30		$ 1.46		$ 0.59		$ 1.49		$ 1.43		$ 1.89		$ 2.04	$ 1.97		$ 4.84	$ 7.34	$ 7.80

[1] Includes $4.0 million of charges consisting of restructuring costs for paper administrative functions and closure costs related to corrugated products facilities ($3.0 million after-tax or $0.03 per diluted share).

[2] Includes $10.0 million of charges related to the impact of Hurricane Laura on our DeRidder, Louisiana mill, including unabsorbed costs related to lost production, excess purchased containerboard and freight costs, repair expenses, rental and supplies costs, and other recovery expenses ($7.6 million after-tax or $0.08 per diluted share) and $3.3 million of charges consisting of closure costs related to corrugated products facilities, substantially all of which relates to the previously announced closure of the San Lorenzo, California facility during the second quarter of 2020 ($2.5 million after-tax or $0.03 per diluted share).

[3] Includes $20.4 million of charges consisting of closure costs related to corrugated products facilities, substantially all of which relates to the previously announced closure of the San Lorenzo, California facility during the second quarter of 2020, partially offset by income related to the sale of a corrugated products facility ($15.3 million after-tax or $0.16 per diluted share) and $6.1 million of incremental, out-of-pocket costs related to COVID-19, including supplies, cleaning and sick pay ($4.6 million after-tax or $0.05 per diluted share). Also includes a non-cash goodwill impairment charge of $55.2 million as a result of the interim quantitative impairment analysis performed on our Paper reporting unit ($55.2 million after-tax or $0.58 per diluted share).

[4] Includes $0.8 million of incremental, out-of-pocket costs related to COVID-19, including supplies, cleaning and sick pay ($0.6 million after-tax or $0.01 per diluted share) and $0.4 million of charges consisting of closure costs related to corrugated products facilities ($0.3 million after-tax or $0.00 per diluted share).

[5] 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).

[6] 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).

[7] 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).