Consolidated Statements of Comprehensive Income (Loss) (unaudited) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 38.6	$ 43.5	$ 82.0	$ 86.4
Other comprehensive income (loss), net of income taxes:
Defined benefit pension and other postretirement plans	(0.2)	(0.4)	(0.4)	(0.7)
Currency translation adjustments	37.9	(8.3)	19.7	(9.7)
Gain (loss) from hedging activities	(5.2)	(0.2)	(3.4)	0.5
Total other comprehensive income (loss)	32.5	(8.9)	15.9	(9.9)
Comprehensive income (loss)	71.1	34.6	97.9	76.5
Less: comprehensive (income) loss attributable to noncontrolling interests	0.2	0.4	0.4	0.4
Comprehensive income (loss) attributable to W. R. Grace & Co. shareholders	$ 71.3	$ 35.0	$ 98.3	$ 76.9