Other Comprehensive Income (Loss) - AOCI (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 353.8	$ 245.9	$ 337.0	$ 263.3
Ending balance	387.3	294.9	387.3	294.9
Defined Benefit Pension and Other Postretirement Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
OCI, before Reclassifications, Net of Tax, Attributable to Parent			0.0	0.0
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent			0.2	0.4
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			0.2	0.9
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(0.1)	(0.2)	(0.2)	(0.4)
Ending balance	0.0	0.5	0.0	0.5
Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
OCI, before Reclassifications, Net of Tax, Attributable to Parent			4.9	19.7
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent			0.0	0.0
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			76.2	41.6
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(6.8)	37.9	4.9	19.7
Ending balance	81.1	61.3	81.1	61.3
Gain (Loss) from Hedging Activities
Accumulated Other Comprehensive Income (Loss) [Line Items]
OCI, before Reclassifications, Net of Tax, Attributable to Parent			(2.1)	20.0
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent			7.0	23.4
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			(8.5)	(2.6)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(6.1)	(5.2)	(9.1)	(3.4)
Ending balance	(17.6)	(6.0)	(17.6)	(6.0)
Other comprehensive income (loss) attributable to W. R. Grace & Co. shareholders
Accumulated Other Comprehensive Income (Loss) [Line Items]
OCI, before Reclassifications, Net of Tax, Attributable to Parent			2.8	39.7
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent			7.2	23.8
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	76.5	23.3	67.9	39.9
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(13.0)	32.5	(4.4)	15.9
Ending balance	$ 63.5	$ 55.8	$ 63.5	$ 55.8