Consolidated Statements of Comprehensive Income (Loss) (unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 15.9	$ 47.1	$ 97.9	$ 133.5
Other comprehensive income (loss), net of income taxes:
Defined benefit pension and other postretirement plans	(0.3)	(0.3)	(0.7)	(1.0)
Currency translation adjustments	(11.1)	(12.1)	8.6	(21.8)
Gain (loss) from hedging activities	2.1	(0.4)	(1.3)	0.1
Total other comprehensive income (loss)	(9.3)	(12.8)	6.6	(22.7)
Comprehensive income (loss)	6.6	34.3	104.5	110.8
Less: comprehensive (income) loss attributable to noncontrolling interests	0.2	0.3	0.6	0.7
Comprehensive income (loss) attributable to W. R. Grace & Co. shareholders	$ 6.8	$ 34.6	$ 105.1	$ 111.5