Accounts Payable And Accrued Expenses (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Schedule Of Accounts Payble And Accrued Expenses [Line Items]
Payables, trade and other	$ 79.4	$ 82.2
Current portion of lease liabilities	5.2
Employment costs	16.5	18.6
Current portion of pension and postretirement liabilities	10.5	10.9
Other	10.0	11.6
Total accounts payable and accrued expenses	148.7	383.3
Knauf [Member]
Schedule Of Accounts Payble And Accrued Expenses [Line Items]
Advance receipt of Knauf proceeds		202.4
Payable to Knauf for purchase price adjustments	1.2
Contingent liability payable to Knauf for adjustments to cash consideration		35.2
Payable to WAVE for receipt of Knauf proceeds	$ 25.9	$ 22.4