Other Comprehensive Income (Loss) - AOCI (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), Net of Tax	$ 1.4	$ 8.6
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	402.2	337.0
Ending balance	383.9	353.8
Defined Benefit Pension and Other Postretirement Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	0.0	0.0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0.1	0.1
Other Comprehensive Income (Loss), Net of Tax	(0.1)	(0.1)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(0.5)	0.2
Ending balance	(0.6)	0.1
Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	2.5	11.7
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0.0	0.0
Other Comprehensive Income (Loss), Net of Tax	2.5	11.7
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	92.7	76.2
Ending balance	95.2	87.9
Gain (Loss) from Hedging Activities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	7.2	6.6
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	8.2	9.6
Other Comprehensive Income (Loss), Net of Tax	(1.0)	(3.0)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(13.4)	(8.5)
Ending balance	(14.4)	(11.5)
Other comprehensive income (loss) attributable to W. R. Grace & Co. shareholders
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	9.7	18.3
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	8.3	9.7
Other Comprehensive Income (Loss), Net of Tax	1.4	8.6
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	78.8	67.9
Ending balance	$ 80.2	$ 76.5