Quarterly Financial Information (Unaudited) (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
Environmental remediation
Provision for environmental remediation									$ 1.7	$ 73.8	$ 24.4
Revenues	$ 504.5	$ 470.5	$ 513.6	$ 469.5	$ 520.0	$ 494.9	$ 485.7	$ 431.5	1,958.1	1,932.1	1,716.5
Gross Profit	204.7	191.0	209.4	188.6	196.3	202.2	198.7	169.5	793.7	766.7	676.1
Net income (loss)	(28.1)	53.8	76.4	24.6	68.9	15.9	38.6	43.4	126.7	166.8	10.4
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
Basic earnings per share:
Net income (loss) (in dollars per share)	$ (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:
Net income (loss) (in dollars per share)	(0.42)	0.80	1.14	0.37	1.03	0.24	0.58	0.64	1.89	2.49	0.16
Dividends per common share	$ 0.27	$ 0.27	$ 0.27	$ 0.27	$ 0.24	$ 0.24	$ 0.24	$ 0.24	$ 1.08	$ 0.96	$ 0.84
Operable Unit 3 [Member]
Environmental remediation
Provision for environmental remediation		$ 70.0				$ 70.0
Spillway [Member]
Environmental remediation
Loss Contingency, Loss in Period				$ 45.0					$ 68.0