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Income Taxes - Narrative (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended		12 Months Ended
Income Taxes - Narrative (Details) - USD ($) $ in Millions	Sep. 30, 2019	Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Effective income tax rate reconciliation, percent	11.00%	3.00%		16.00%	8.00%
Unrecognized tax benefits reduction resulting from lapse of applicable statute of limitations	$ 1.2			$ 1.2
Investments, Owned, Federal Income Tax Note [Line Items]
Impairment loss (Note 12)	$ 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
Investments, Owned, Federal Income Tax Note [Line Items]
Impairment loss (Note 12)		$ 2.0	$ 32.0		$ 34.0	$ 31.1