Segment Results (Schedule Of Net Sales to External Customers) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Segment Reporting Information [Line Items]
Net sales to external customers	$ 382.7	$ 326.3
Cost of goods sold	232.8	202.0
Gross profit (loss)	149.9	124.3
Selling, general and administrative expenses	78.0	65.4
Equity (earnings) loss from unconsolidated affiliates, net	(26.6)	(27.2)
Segment operating income (loss)	98.5	86.1
Unallocated Corporate [Member]
Segment Reporting Information [Line Items]
Cost of goods sold	0.4	0.4
Gross profit (loss)	(0.4)	(0.4)
Selling, general and administrative expenses	0.2	0.2
Equity (earnings) loss from unconsolidated affiliates, net	0.2	0.2
Segment operating income (loss)	(0.8)	(0.8)
Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales to external customers	382.7	326.3
Cost of goods sold	232.8	202.0
Gross profit (loss)	149.9	124.3
Selling, general and administrative expenses	78.0	65.4
Equity (earnings) loss from unconsolidated affiliates, net	(26.6)	(27.2)
Segment operating income (loss)	98.5	86.1
Depreciation and amortization	29.3	24.3
Purchases of property, plant and equipment	19.1	14.7
Mineral Fiber [Member]
Segment Reporting Information [Line Items]
Net sales to external customers	245.1	239.6
Mineral Fiber [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales to external customers	245.1	239.6
Cost of goods sold	148.0	145.7
Gross profit (loss)	97.1	93.9
Selling, general and administrative expenses	39.4	42.1
Equity (earnings) loss from unconsolidated affiliates, net	(26.8)	(27.4)
Segment operating income (loss)	84.5	79.2
Depreciation and amortization	20.8	19.9
Purchases of property, plant and equipment	14.6	11.6
Architectural Specialties [Member]
Segment Reporting Information [Line Items]
Net sales to external customers	137.6	86.7
Architectural Specialties [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales to external customers	137.6	86.7
Cost of goods sold	84.4	55.9
Gross profit (loss)	53.2	30.8
Selling, general and administrative expenses	38.4	23.1
Segment operating income (loss)	14.8	7.7
Depreciation and amortization	8.5	4.4
Purchases of property, plant and equipment	$ 4.5	$ 3.1