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, 2015		Sep. 30, 2015		Jun. 30, 2015		Mar. 31, 2015		Dec. 31, 2014	Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Numerator:
Net income	$ 104.3	[1]	$ 127.8	[2]	$ 114.0	[3]	$ 90.8	[4]	$ 98.5	$ 104.4	[5]	$ 99.6	[6]	$ 90.1	[7]	$ 436.8	$ 392.6	$ 441.3
Less: distributed and undistributed earnings allocated to participating securities																(5.2)	(5.7)	0.0
Net income attributable to common shareholders																$ 431.6	$ 386.9	$ 441.3
Denominator:
Weighted average basic common shares outstanding																96.6	97.0	96.6
Effect of dilutive securities																0.1	0.1	0.9
Diluted common shares outstanding																96.7	97.1	97.5
Basic income per common share	$ 1.07		$ 1.31		$ 1.16		$ 0.92		$ 1.00	$ 1.06		$ 1.01		$ 0.92		$ 4.47	$ 3.99	$ 4.57
Diluted income per common share	$ 1.07	[1]	$ 1.31	[2]	$ 1.16	[3]	$ 0.92	[4]	$ 1.00	$ 1.06	[5]	$ 1.01	[6]	$ 0.92	[7]	$ 4.47	$ 3.99	$ 4.52

[1] Includes $3.5 million of income from DeRidder restructuring ($2.2 million after-tax or $0.02 per diluted share) and $3.8 million of integration-related and other costs ($2.6 million after-tax or $0.03 per diluted share).

[2] Includes $3.8 million of income from DeRidder restructuring ($2.3 million after-tax or $0.02 per diluted share) and $2.4 million of integration-related and other costs ($1.7 million after-tax or $0.02 per diluted share). Also includes $6.7 million gain from the sale of our paper mill site at St. Helens, Oregon ($4.4 million after tax or $0.05 per diluted share).

[3] Includes $1.0 million of income from DeRidder restructuring ($0.7 million after-tax or $0.01 per diluted share) and $3.7 million of integration-related and other costs ($2.3 million after-tax or $0.03 per diluted share). Gross profit has been revised to correct an error in the previous presentation totaling $6.0 million reclassified from "Selling, general, and administrative expenses" to "Cost of Sales" for the three months ended June 30, 2015.

[4] Includes $10.3 million of DeRidder restructuring charges ($6.6 million after-tax or $0.07 per diluted share) and $3.5 million of integration-related and other costs ($2.2 million after-tax or $0.02 per diluted share). Also includes a $3.6 million tax credit from the State of Louisiana related to our recent capital investment and the jobs retained at the DeRidder, Louisiana mill.

[5] Includes $26.0 million of DeRidder restructuring charges ($16.6 million after-tax or $0.17 per diluted share) and $4.5 million of integration-related and other costs ($2.9 million after-tax or $0.03 per diluted share).

[6] Includes $17.8 million of DeRidder restructuring charges ($11.2 million after-tax or $0.12 per diluted share) and $4.9 million of integration-related and other costs ($3.0 million after-tax or $0.03 per diluted share).

[7] Includes $17.6 million of costs accrued for the settlement of the Kleen Products LLC v Packaging Corp. of America et al class action lawsuit ($11.2 million after-tax or $0.11 per diluted share), $4.1 million of integration-related costs ($2.6 million after-tax or $0.03 per diluted share), and $4.0 million of DeRidder restructuring charges ($2.6 million after-tax or $0.02 per diluted share).