Other Comprehensive Income (Loss) - OCI Components (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pre-Tax Amount	$ (0.1)	$ (0.1)
Tax Benefit/ (Expense)	0.0	0.0
After-Tax Amount	(0.1)	(0.1)
Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pre-Tax Amount	3.9	12.9
Tax Benefit/ (Expense)	(1.4)	(1.2)
After-Tax Amount	2.5	11.7
Gain (Loss) from Hedging Activities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pre-Tax Amount	(1.4)	(4.3)
Tax Benefit/ (Expense)	0.4	1.3
After-Tax Amount	(1.0)	(3.0)
Other comprehensive income (loss) attributable to W. R. Grace & Co. shareholders
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pre-Tax Amount	2.4	8.5
Tax Benefit/ (Expense)	(1.0)	0.1
After-Tax Amount	$ 1.4	$ 8.6