Nature of Operations (Schedule of Net Sales to External Customers) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Net sales	$ 1,106.6	$ 936.9	$ 1,038.1
Equity (earnings) from joint venture	(87.7)	(64.0)	(96.6)
Segment operating income (loss)	260.0	254.8	317.4
Segment assets	1,710.0	1,718.5	1,493.3
Depreciation and amortization	96.5	84.0	72.1
Investment in joint venture	50.0	41.2	58.5
Purchases of property, plant and equipment	79.8	55.4	68.3	[1]
Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Net sales	818.5	726.0	826.6
Equity (earnings) from joint venture	87.7	(64.0)	(96.6)
Segment operating income (loss)	261.2	218.7	289.6
Segment assets	1,133.9	1,101.1	1,139.9
Depreciation and amortization	69.9	71.8	62.9
Investment in joint venture	50.0	41.2	58.5
Purchases of property, plant and equipment	64.8	45.5	59.5	[1]
Architectural Specialties [Member]
Segment Reporting Information [Line Items]
Net sales	288.1	210.9	211.5
Segment operating income (loss)	4.2	22.3	35.9
Segment assets	366.3	357.7	161.8
Depreciation and amortization	26.6	12.2	8.8
Purchases of property, plant and equipment	15.0	9.9	8.8	[1]
Unallocated Corporate [Member]
Segment Reporting Information [Line Items]
Segment operating income (loss)	(5.4)	13.8	(8.1)
Segment assets	$ 209.8	$ 259.7	191.6
Depreciation and amortization			$ 0.4

[1] Total differs from our Consolidated Statement of Cash Flows by the amount that was classified as discontinued operations. See Note 6 for additional details.