Accounts Payable And Accrued Expenses (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Schedule Of Accounts Payble And Accrued Expenses [Line Items]
Payables, trade and other	$ 81.3	$ 79.4
Employment costs	17.6	16.5
Current portion of pension and postretirement liabilities	9.5	10.5
Other	16.7	10.0
Total accounts payable and accrued expenses	136.5	143.5
Knauf [Member]
Schedule Of Accounts Payble And Accrued Expenses [Line Items]
Payable to Knauf for purchase price adjustments	$ 11.4	1.2
WAVE [Member]
Schedule Of Accounts Payble And Accrued Expenses [Line Items]
Payable to WAVE for receipt of Knauf proceeds		$ 25.9