Transactions with WestRock and related-parties - Summary of Allocated Cost (Details) - Affiliated Entity - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Income (Loss)
Total allocated cost	$ 0.0	$ 19.4	$ 41.1
Cost of sales
Operating Income (Loss)
Total allocated cost	0.0	5.7	10.3
Selling, general and administrative expenses
Operating Income (Loss)
Total allocated cost	0.0	6.5	17.3
Interest expense, net
Operating Income (Loss)
Total allocated cost	$ 0.0	$ 7.2	$ 13.5