Segment Information - Reportable Segment Data (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) segment	Jun. 30, 2019 USD ($)
Operating segment information
Number of reportable segments | segment			2
Net Sales
Revenues	$ 418.7	$ 513.6	$ 840.2	$ 983.1
Adjusted EBIT
Total	64.1	127.3	146.4	232.0
Reconciliation of operating segment data to financial statements
Restructuring and Repositioning expenses attributable to W. R. Grace & Co. shareholders	21.4	6.4	24.1	8.7
Adjusted EBIT inventory write-off	(19.7)	(3.6)	(19.7)	(3.6)
Costs related to legacy matters	(2.8)	(1.5)	(5.5)	(48.4)
Third-party acquisition-related costs	(2.0)	(1.0)	(3.5)	(1.3)
Taxes and interest included in equity in earnings of unconsolidated affiliate	(0.2)	(0.6)	(0.2)	(0.9)
Interest expense, net	(18.9)	(19.2)	(36.6)	(38.5)
Net Income (Loss) Attributable to Noncontrolling Interest	(2.3)	0.2	(2.2)	0.1
Income (loss) before income taxes	(3.2)	95.2	$ 54.6	130.7
Catalysts Technologies
Operating segment information
Number of reportable segments | segment			1
Number of operating segments | segment			2
Net Sales
Revenues	309.0	395.7	$ 617.0	745.4
Materials Technologies
Net Sales
Revenues	109.7	117.9	223.2	237.7
Corporate
Adjusted EBIT
Operating income	(16.7)	(18.0)	(32.3)	(34.2)
Certain pension costs	(3.5)	(4.6)	(6.6)	(9.4)
Operating Segments | Catalysts Technologies
Net Sales
Revenues	309.0	395.7	617.0	745.4
Adjusted EBIT
Operating income	71.7	125.8	153.7	227.5
Operating Segments | Materials Technologies
Net Sales
Revenues	109.7	117.9	223.2	237.7
Adjusted EBIT
Operating income	$ 12.6	$ 24.1	$ 31.6	$ 48.1