Sale of Brattleboro Mill and Impairment Loss (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Impairment loss	$ 0.0	$ 2.0		$ 0.0	$ 34.0
Brattleboro Mill And Associated Research And Office Facilities | Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Impairment loss		$ 2.0	$ 32.0		$ 34.0	$ 31.1
Cash proceeds from sale of mill						5.0
Fine Paper and Packaging | Brattleboro Mill And Associated Research And Office Facilities | Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Impairment loss			25.1			24.4
Technical Products | Brattleboro Mill And Associated Research And Office Facilities | Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Impairment loss			1.1			1.1
Other | Brattleboro Mill And Associated Research And Office Facilities | Disposal Group, Held-for-sale or Disposed of by Sale, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Impairment loss			$ 5.8			$ 5.6