Other Comprehensive Income (Loss) - AOCI (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			$ 263.3	$ 372.4
Total other comprehensive income (loss)	$ 32.5	$ (8.9)	15.9	(9.9)
Ending balance	294.9	405.6	294.9	405.6
Defined Benefit Pension and Other Postretirement Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			0.9	2.2
Other comprehensive income (loss) before reclassifications			0.0	0.0
Amounts reclassified from accumulated other comprehensive income (loss)			(0.4)	(0.7)
Total other comprehensive income (loss)			(0.4)	(0.7)
Ending balance	0.5	1.5	0.5	1.5
Currency Translation Adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			41.6	67.6
Other comprehensive income (loss) before reclassifications			19.7	(9.7)
Amounts reclassified from accumulated other comprehensive income (loss)			0.0	0.0
Total other comprehensive income (loss)			19.7	(9.7)
Ending balance	61.3	57.9	61.3	57.9
Gain (Loss) from Hedging Activities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			(2.6)	(3.4)
Other comprehensive income (loss) before reclassifications			20.0	(0.7)
Amounts reclassified from accumulated other comprehensive income (loss)			(23.4)	1.2
Total other comprehensive income (loss)			(3.4)	0.5
Ending balance	(6.0)	(2.9)	(6.0)	(2.9)
Other comprehensive income (loss) attributable to W. R. Grace & Co. shareholders
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			39.9	66.4
Other comprehensive income (loss) before reclassifications			39.7	(10.4)
Amounts reclassified from accumulated other comprehensive income (loss)			(23.8)	0.5
Total other comprehensive income (loss)			15.9	(9.9)
Ending balance	$ 55.8	$ 56.5	$ 55.8	$ 56.5