Nature of Operations (Schedule of Net Sales to External Customers) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Segment Reporting Information [Line Items]
Net sales	$ 1,233.1	$ 1,106.6	$ 936.9
Equity (earnings) from joint venture	(77.6)	(87.7)	(64.0)
Segment operating income (loss)	278.7	260.0	254.8
Segment assets	1,687.2	1,710.0	1,718.5
Depreciation and amortization	83.7	96.5	84.0
Investment in joint venture	23.9	50.0	41.2
Purchases of property, plant and equipment	74.8	79.8	55.4
Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Net sales	887.4	818.5	726.0
Equity (earnings) from joint venture	(77.6)	(87.7)	(64.0)
Segment operating income (loss)	260.9	261.2	218.7
Segment assets	1,096.9	1,133.9	1,101.1
Depreciation and amortization	69.5	69.9	71.8
Investment in joint venture	23.9	50.0	41.2
Purchases of property, plant and equipment	63.8	64.8	45.5
Architectural Specialties [Member]
Segment Reporting Information [Line Items]
Net sales	345.7	288.1	210.9
Segment operating income (loss)	21.7	4.2	22.3
Segment assets	387.5	366.3	357.7
Depreciation and amortization	14.2	26.6	12.2
Purchases of property, plant and equipment	11.0	15.0	9.9
Unallocated Corporate [Member]
Segment Reporting Information [Line Items]
Segment operating income (loss)	(3.9)	(5.4)	13.8
Segment assets	$ 202.8	$ 209.8	$ 259.7