Nature of Operations (Schedule of Net Sales to External Customers) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting Information [Line Items]
Net sales	$ 1,295.2	$ 1,233.1	$ 1,106.6
Equity (earnings) from joint venture	(89.3)	(77.6)	(87.7)
Segment operating income (loss)	323.7	278.7	260.0
Segment assets	1,672.4	1,687.2	1,710.0
Depreciation and amortization	89.2	83.7	96.5
Investment in joint venture	17.4	23.9	50.0
Purchases of property, plant and equipment	83.8	74.8	79.8
Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Net sales	932.4	887.4	818.5
Equity (earnings) from joint venture	(89.3)	(77.6)	(87.7)
Segment operating income (loss)	285.7	260.9	261.2
Segment assets	1,091.9	1,096.9	1,133.9
Depreciation and amortization	75.3	69.5	69.9
Investment in joint venture	17.4	23.9	50.0
Purchases of property, plant and equipment	67.2	63.8	64.8
Architectural Specialties [Member]
Segment Reporting Information [Line Items]
Net sales	362.8	345.7	288.1
Segment operating income (loss)	40.9	21.7	4.2
Segment assets	421.1	387.5	366.3
Depreciation and amortization	13.9	14.2	26.6
Purchases of property, plant and equipment	16.6	11.0	15.0
Unallocated Corporate [Member]
Segment Reporting Information [Line Items]
Segment operating income (loss)	(2.9)	(3.9)	(5.4)
Segment assets	$ 159.4	$ 202.8	$ 209.8