Severance and Related Costs (Narrative) (Details) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017 USD ($) employee	Dec. 31, 2016 USD ($) employee	Dec. 31, 2015 USD ($)
Mineral Fiber [Member]
Severance and Related Costs [Line Items]
Positions eliminated | employee	126
Mineral Fiber [Member] | Cost Of Goods Sold [Member]
Severance and Related Costs [Line Items]
Severance and related costs | $	$ 3.3
Mineral Fiber and Architectural Specialties Segments [Member]
Severance and Related Costs [Line Items]
Positions eliminated | employee	18
Mineral Fiber and Architectural Specialties Segments [Member] | Selling, General and Administrative (SG&A) Expense [Member]
Severance and Related Costs [Line Items]
Severance and related costs | $	$ 1.3
Unallocated Corporate Segment [Member]
Severance and Related Costs [Line Items]
Severance and related costs | $		$ 2.4	$ 5.3
Positions eliminated | employee		30