Other Comprehensive Income (Loss) - AOCI (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), Net of Tax	$ (3.3)	$ 27.0	$ (8.4)	$ 1.4	$ 23.7	$ (7.0)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, December 31, 2020	308.3	234.5	383.9	402.2	234.5	402.2
Balance, June 30, 2021	350.0	308.3	349.6	383.9	350.0	349.6
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)	(0.2)
Other Comprehensive Income (Loss), Net of Tax					(0.1)	(0.2)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, December 31, 2020		(0.9)		(0.5)	(0.9)	(0.5)
Balance, June 30, 2021	(1.0)		(0.7)		(1.0)	(0.7)
Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax					18.8	(5.0)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax					0.0	0.0
Other Comprehensive Income (Loss), Net of Tax					18.8	(5.0)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, December 31, 2020		42.8		92.7	42.8	92.7
Balance, June 30, 2021	61.6		87.7		61.6	87.7
Gain (Loss) from Hedging Activities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax					16.6	2.2
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax					(11.6)	(4.0)
Other Comprehensive Income (Loss), Net of Tax					5.0	(1.8)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, December 31, 2020		(12.6)		(13.4)	(12.6)	(13.4)
Balance, June 30, 2021	(7.6)		(15.2)		(7.6)	(15.2)
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					35.4	(2.8)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax					(11.7)	(4.2)
Other Comprehensive Income (Loss), Net of Tax	(3.3)	27.0	(8.4)	1.4	23.7	(7.0)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, December 31, 2020	56.3	29.3	80.2	78.8	29.3	78.8
Balance, June 30, 2021	$ 53.0	$ 56.3	$ 71.8	$ 80.2	$ 53.0	$ 71.8

W R GRACE & CO 10-Q