Goodwill and Other Intangible Assets - Changes in carrying value of goodwill (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Goodwill [Roll Forward]
Goodwill, net, beginning balance	$ 85,300,000	$ 85,300,000	$ 70,400,000
Goodwill acquired in the Coldenhove Acquisition			10,000,000
Foreign currency translation		(2,300,000)	4,900,000
Adjustment of goodwill acquired		1,100,000
Impairment related to the Brattleboro mill and associated office and research facilities			0	$ 0
Goodwill, net, ending balance		$ 84,000,000	85,300,000	70,400,000
Minimum
Goodwill [Line Items]
Useful lives		10 years
Maximum
Goodwill [Line Items]
Useful lives		15 years
Technical Products
Goodwill [Roll Forward]
Goodwill gross, beginning balance	128,300,000	$ 128,300,000	107,400,000
Accumulated Impairment Loss, beginning of period	(49,600,000)	(49,600,000)	(43,600,000)
Goodwill, net, beginning balance	78,700,000	78,700,000	63,800,000
Goodwill acquired in the Coldenhove Acquisition			10,000,000
Foreign currency translation gross impact		(4,500,000)	10,900,000
Foreign currency translation, Accumulated Impairment Losses Impact		2,200,000	(6,000,000)
Foreign currency translation		(2,300,000)	4,900,000
Adjustment of goodwill acquired		1,100,000
Goodwill gross, ending balance		124,900,000	128,300,000	107,400,000
Accumulated Impairment Loss, end of period		(47,400,000)	(49,600,000)	(43,600,000)
Goodwill, net, ending balance		77,500,000	78,700,000	63,800,000
Fine Paper and Packaging
Goodwill [Roll Forward]
Goodwill gross, beginning balance	6,200,000	6,200,000	6,200,000
Goodwill, net, beginning balance	6,200,000	6,200,000	6,200,000
Goodwill gross, ending balance		6,200,000	6,200,000	6,200,000
Goodwill, net, ending balance		6,200,000	6,200,000	6,200,000
Other
Goodwill [Roll Forward]
Goodwill gross, beginning balance	400,000	400,000	400,000
Goodwill, net, beginning balance	400,000	400,000	400,000
Impairment related to the Brattleboro mill and associated office and research facilities		(100,000)
Goodwill gross, ending balance		400,000	400,000	400,000
Accumulated Impairment Loss, end of period		(100,000)
Goodwill, net, ending balance		300,000	400,000	$ 400,000
Goodwill Other Than Brattleboro Mill
Goodwill [Roll Forward]
Impairment related to the Brattleboro mill and associated office and research facilities		0
W.A. Sanders Coldenhove Holding B.V.
Goodwill [Roll Forward]
Goodwill, net, beginning balance	10,000,000	10,000,000
Adjustment of goodwill acquired	$ 1,100,000
Goodwill, net, ending balance			$ 10,000,000
W.A. Sanders Coldenhove Holding B.V. | Technical Products
Goodwill [Roll Forward]
Adjustment of goodwill acquired		1,100,000
Brattleboro Mill and Associated Research and Office Facilities
Goodwill [Roll Forward]
Impairment related to the Brattleboro mill and associated office and research facilities		$ (100,000)