Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Feb. 08, 2017	Feb. 05, 2015
Numerators
Net income (loss) attributable to W. R. Grace & Co. shareholders	$ (28.3)	$ 53.7	$ 76.2	$ 24.7	$ 69.1	$ 16.1	$ 38.8	$ 43.6	$ 126.3	$ 167.6	$ 11.2
Denominators
Weighted average common shares-basic calculation									66,800,000	67,200,000	68,100,000
Dilutive effect of employee stock options (in shares)									100,000	100,000	100,000
Weighted average common shares-diluted calculation									66,900,000	67,300,000	68,200,000
Basic earnings per share attributable to W. R. Grace & Co. shareholders
Net income (loss)	$ (0.42)	$ 0.81	$ 1.14	$ 0.37	$ 1.03	$ 0.24	$ 0.58	$ 0.64	$ 1.89	$ 2.49	$ 0.16
Diluted earnings per share attributable to W. R. Grace & Co. shareholders
Net income (loss)	$ (0.42)	$ 0.80	$ 1.14	$ 0.37	$ 1.03	$ 0.24	$ 0.58	$ 0.64	$ 1.89	$ 2.49	$ 0.16
Stock options excluded from computation of diluted earnings per share (in shares)									1,000,000.0	1,700,000	1,500,000
Share Repurchase Program [Line Items]
Stock Repurchase Program, Authorized Amount												$ 250.0	$ 500.0
Shares repurchased									409,769	1,171,141	935,435
Payments for Repurchase of Common Stock									$ 29.8	$ 80.0	$ 65.0
Stock Repurchase Program, Remaining Authorized Repurchase Amount	$ 109.1								$ 109.1