Other Comprehensive Income (Loss) - OCI Components (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Amortization of net prior service credit included in net periodic benefit cost
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pre-Tax Amount	$ (0.2)	$ (0.4)
Tax Benefit/ (Expense)	0.0	0.1
After-Tax Amount	(0.2)	(0.3)
Amortization of net deferred actuarial loss included in net periodic benefit cost
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
Amortization of net deferred actuarial loss included in net periodic benefit cost
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pre-Tax Amount	(0.1)	(0.3)
Tax Benefit/ (Expense)	0.0	0.1
After-Tax Amount	(0.1)	(0.2)
Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pre-Tax Amount	12.9	(20.8)
Tax Benefit/ (Expense)	(1.2)	2.6
After-Tax Amount	11.7	(18.2)
Gain (Loss) from Hedging Activities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pre-Tax Amount	(4.3)	2.2
Tax Benefit/ (Expense)	1.3	(0.4)
After-Tax Amount	(3.0)	1.8
Other comprehensive income (loss) attributable to W. R. Grace & Co. shareholders
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pre-Tax Amount	8.5	(18.9)
Tax Benefit/ (Expense)	0.1	2.3
After-Tax Amount	$ 8.6	$ (16.6)