Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2014	Mar. 31, 2014	Jun. 30, 2015	Jun. 30, 2014
Defined benefit pension and other postretirement plans, Pre-Tax Amount
Other Comprehensive Income, Effect of Postretirement Plan Changes, Pre-tax, Net			$ 41.9			$ 41.9
Other comprehensive income - other changes in funded status, pre tax	$ (0.4)				$ (0.4)
Amortization of net deferred actuarial loss included in net periodic benefit cost, Pre-Tax Amount	0.1		(7.9)		0.3	(7.9)
Amortization of net prior service cost included in net periodic benefit cost, Pre-Tax Amount	(0.9)		(0.2)		(1.8)	(0.1)
Other Comprehensive Income, Defined Benefit Plan, Other Changes in Funded Status Recognized in Net Periodic Benefit Cost, before Tax			(0.1)			(0.3)
Benefit plans, net, Pre-Tax Amount	(1.2)		33.7		(1.9)	33.6
Currency translation adjustments, pre-tax amount	(0.7)		7.3		(11.5)	5.2
Loss from hedging activities, pre-tax amount	2.5		(3.7)		(0.4)	(2.6)
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, before Tax, Portion Attributable to Parent			(0.9)			(0.8)
Other comprehensive income attributable to W. R. Grace and Co. shareholders, Pre-Tax Amount	0.6		36.4		(13.8)	35.4
Defined benefit pension and other postretirement plans, Tax Benefit/(Expense)
Other Comprehensive Income, Effect of Postretirement Plan Changes, Tax			14.7			14.7
Other comprehensive income - other changes in funded status, tax	0.1				0.1
Amortization of net deferred actuarial gain included in net periodic benefit cost, tax benefit/(expense)	0.0		2.8		(0.1)	2.8
Amortization of net prior service credit included in net periodic benefit cost, tax benefit/(expense)	0.3		0.0		0.7	0.0
Other Comprehensive Income, Defined Benefit Plan, Other Changes in Funded Status Recognized in Net Periodic Benefit Cost Tax Effect			0.1			0.1
Benefit plans, net, Tax Benefit/(Expense)	0.4		(11.8)		0.7	(11.8)
Currency translation adjustments, tax benefit/(expense)	0.0		0.0		0.0	0.0
Loss from hedging activities, tax benefit/(expense)	(0.9)		1.3		0.1	0.9
Other Comprehensive Income (Loss), Available-for-sale Securities, Tax, Portion Attributable to Parent			0.3			0.3
Other comprehensive income attributable to W. R. Grace and Co. shareholders, Tax Benefit/(Expense)	(0.5)		(10.2)		0.8	(10.6)
Defined benefit pension and other postretirement plans, After-Tax Amount:
Other Comprehensive Income, Effect of Postretirement Plan Changes, Net of Tax			27.2			27.2
Other comprehensive income - other changes in funded status, net of tax	(0.3)				(0.3)
Amortization of net deferred actuarial loss included in net periodic benefit cost, After-Tax Amount	0.1		(5.1)		0.2	(5.1)
Amortization of net prior service credit included in net periodic benefit cost, After-Tax Amount	(0.6)		(0.2)		(1.1)	(0.1)
Other Comprehensive Income, Defined Benefit Plan, Other Changes in Funded Status Recognized in Net Periodic Benefit Cost Net of Tax			0.0			(0.2)
Benefit plans, net, after-tax amount	(0.8)	$ (1.2)	21.9	$ 21.8	(1.2)	21.8
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax, Portion Attributable to Parent	(0.7)	(11.5)	7.3	5.2	(11.5)	5.2
Gain (loss) from hedging activities, net of income taxes	1.6	(0.3)	(2.4)	(1.7)	(0.3)	(1.7)
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax, Portion Attributable to Parent	0.0		(0.6)	(0.5)	0.0	(0.5)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	$ 0.1	$ (13.0)	$ 26.2	$ 24.8	$ (13.0)	$ 24.8