Other Comprehensive Income (Loss) - OCI Components (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 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)	$ (0.7)	$ (1.2)
Tax Benefit/ (Expense)	0.1	0.1	0.2	0.3
After-Tax Amount	(0.1)	(0.3)	(0.5)	(0.9)
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	0.3	0.3
Tax Benefit/ (Expense)	(0.1)	(0.1)	(0.1)	(0.1)
After-Tax Amount	0.0	0.0	0.2	0.2
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)	(0.4)	(0.9)
Tax Benefit/ (Expense)	0.0	0.0	0.1	0.2
After-Tax Amount	(0.1)	(0.3)	(0.3)	(0.7)
Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pre-Tax Amount	24.2	(11.7)	30.2	8.5
Tax Benefit/ (Expense)	(1.8)	0.6	(2.9)	0.1
After-Tax Amount	22.4	(11.1)	27.3	8.6
Gain (Loss) from Hedging Activities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pre-Tax Amount	(0.7)	2.7	(13.7)	(3.5)
Tax Benefit/ (Expense)	0.2	(0.6)	4.1	2.2
After-Tax Amount	(0.5)	2.1	(9.6)	(1.3)
Other comprehensive income (loss) attributable to W. R. Grace & Co. shareholders
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pre-Tax Amount	23.4	(9.3)	16.1	4.1
Tax Benefit/ (Expense)	(1.6)	0.0	1.3	2.5
After-Tax Amount	$ 21.8	$ (9.3)	$ 17.4	$ 6.6