Business and Basis of Presentation (Narrative) (Details)					3 Months Ended
	Mar. 04, 2019 Facility	Aug. 16, 2018 Facility	Jul. 18, 2018 USD ($)	May 31, 2018 Facility	Mar. 31, 2019 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Nov. 17, 2017 USD ($)
Business And Basis Of Presentation [Line Items]
Potential adjustments to the purchase price consideration			$ 35,000,000
Potential adjustments to the purchase price consideration			20,000,000
Account payable and accrued expenses related to investment in joint venture					$ 390,900,000		$ 383,300,000
Lease right-of-use assets					28,800,000
Lease liabilities					28,800,000
Finance lease, liabilities					0
Accounting Standards Update 2016-02 [Member]
Business And Basis Of Presentation [Line Items]
Lease right-of-use assets					28,800,000
Lease liabilities					$ 28,800,000
August 1, 2018 [Member] | Knauf [Member]
Business And Basis Of Presentation [Line Items]
Receivable from sale of business			250,000,000
Proceeds from sale of business						$ 250,000,000
September 15, 2018 [Member] | Knauf [Member]
Business And Basis Of Presentation [Line Items]
Receivable from sale of business			$ 80,000,000
Proceeds from sale of business						80,000,000
EMEA and Pacific Rim Business [Member]
Business And Basis Of Presentation [Line Items]
Consideration to be received in connection with sale of businesses								$ 330,000,000
Architectural Components Group, Inc. [Member]
Business And Basis Of Presentation [Line Items]
Number of manufacturing facility | Facility	1
Steel Ceilings, Inc. [Member]
Business And Basis Of Presentation [Line Items]
Number of manufacturing facility | Facility		1
Plasterform, Inc. [Member]
Business And Basis Of Presentation [Line Items]
Number of manufacturing facility | Facility				1
WAVE [Member]
Business And Basis Of Presentation [Line Items]
Equity interest percentage					50.00%
Payment to WAVE from Knauf proceeds						$ 70,000,000
Account payable and accrued expenses related to investment in joint venture					$ 22,400,000
WAVE [Member] | Worthington [Member]
Business And Basis Of Presentation [Line Items]
Dividend received from joint venture					$ 35,000,000