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Equity Investment (Narrative) (Details) - USD ($)
$ in Millions
1 Months Ended 3 Months Ended 9 Months Ended
Oct. 30, 2018
Sep. 30, 2018
Sep. 30, 2018
Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]        
Payment to WAVE from Knauf proceeds     $ 70.0  
Account payable and accrued expenses related to investment in joint venture   $ 366.5 $ 366.5 $ 108.4
WAVE [Member]        
Schedule of Equity Method Investments [Line Items]        
Equity interest percentage   50.00% 50.00%  
Payment to WAVE from Knauf proceeds   $ 70.0 $ 70.0  
Dividend received from joint venture   35.0    
Account payable and accrued expenses related to investment in joint venture   $ 22.4 $ 22.4  
WAVE [Member] | Subsequent Event [Member]        
Schedule of Equity Method Investments [Line Items]        
Dividend received from joint venture $ 25.0