Nature of Operations (Schedule of Net Sales to External Customers) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Segment Reporting Information [Line Items]
Net sales to external customers	$ 1,620.8	$ 1,445.7	$ 1,295.2
Cost of goods sold	962.1	864.1	798.2
Gross profit (loss)	658.7	581.6	497.0
Selling, general and administrative expenses	339.5	308.5	262.5
Loss related to change in fair value of contingent consideration	1.4	1.6	0.1
(Gain) loss on sales of fixed assets, net	(0.8)	0.6	0.0
Equity earnings from unconsolidated affiliates, net	(112.3)	(103.4)	(89.3)
Segment operating income (loss)	430.9	374.3	323.7
Segment assets	1,924.7	1,842.7
Investment in unconsolidated affiliates	26.6	27.2
Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Net sales to external customers	1,030.7	986.0	932.4
Architectural Specialties [Member]
Segment Reporting Information [Line Items]
Net sales to external customers	590.1	459.7	362.8
Segment Reporting, Reconciling Item, Corporate Nonsegment [Member] | Unallocated Corporate [Member]
Segment Reporting Information [Line Items]
Cost of goods sold	1.4	1.6	1.7
Gross profit (loss)	(1.4)	(1.6)	(1.7)
Selling, general and administrative expenses	1.0	1.0	1.2
Equity earnings from unconsolidated affiliates, net	0.9	0.9
Segment operating income (loss)	(3.3)	(3.5)	(2.9)
Segment assets	216.4	176.7	159.4
Investment in unconsolidated affiliates	3.7	4.6
Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales to external customers	1,620.8	1,445.7	1,295.2
Cost of goods sold	962.1	864.1	798.2
Gross profit (loss)	658.7	581.6	497.0
Selling, general and administrative expenses	339.5	308.5	262.5
Loss related to change in fair value of contingent consideration	1.4	1.6	0.1
(Gain) loss on sales of fixed assets, net	(0.8)	0.6
Equity earnings from unconsolidated affiliates, net	(112.3)	(103.4)	(89.3)
Segment operating income (loss)	430.9	374.3	323.7
Segment assets	1,924.7	1,842.7	1,672.4
Investment in unconsolidated affiliates	26.6	27.2	17.4
Depreciation and amortization	120.4	103.2	89.2
Purchases of property, plant and equipment	109.4	82.8	83.8
Operating Segments [Member] | Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Net sales to external customers	1,030.7	986.0	932.4
Cost of goods sold	603.3	586.2	574.1
Gross profit (loss)	427.4	399.8	358.3
Selling, general and administrative expenses	178.9	180.8	161.9
Loss related to change in fair value of contingent consideration	0.6	0.2
(Gain) loss on sales of fixed assets, net	(0.9)	0.6
Equity earnings from unconsolidated affiliates, net	(113.2)	(104.3)	(89.3)
Segment operating income (loss)	362.0	322.5	285.7
Segment assets	1,095.6	1,063.8	1,091.9
Investment in unconsolidated affiliates	22.9	22.6	17.4
Depreciation and amortization	86.7	80.2	75.3
Purchases of property, plant and equipment	86.9	64.3	67.2
Operating Segments [Member] | Architectural Specialties [Member]
Segment Reporting Information [Line Items]
Net sales to external customers	590.1	459.7	362.8
Cost of goods sold	357.4	276.3	222.4
Gross profit (loss)	232.7	183.4	140.4
Selling, general and administrative expenses	159.6	126.7	99.4
Loss related to change in fair value of contingent consideration	0.8	1.4	0.1
(Gain) loss on sales of fixed assets, net	0.1
Segment operating income (loss)	72.2	55.3	40.9
Segment assets	612.7	602.2	421.1
Depreciation and amortization	33.7	23.0	13.9
Purchases of property, plant and equipment	$ 22.5	$ 18.5	$ 16.6