Other Comprehensive Income (Loss) - AOCI (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 337.0	$ 263.3
Total other comprehensive income (loss)	8.6	(16.6)
Ending balance	353.8	245.9
Defined Benefit Pension and Other Postretirement Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	0.2	0.9
Other comprehensive income (loss) before reclassifications	0.0	0.0
Amounts reclassified from accumulated other comprehensive income (loss)	(0.1)	(0.2)
Total other comprehensive income (loss)	(0.1)	(0.2)
Ending balance	0.1	0.7
Currency Translation Adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	76.2	41.6
Other comprehensive income (loss) before reclassifications	11.7	(18.2)
Amounts reclassified from accumulated other comprehensive income (loss)	0.0	0.0
Total other comprehensive income (loss)	11.7	(18.2)
Ending balance	87.9	23.4
Gain (Loss) from Hedging Activities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(8.5)	(2.6)
Other comprehensive income (loss) before reclassifications	6.6	(5.4)
Amounts reclassified from accumulated other comprehensive income (loss)	(9.6)	7.2
Total other comprehensive income (loss)	(3.0)	1.8
Ending balance	(11.5)	(0.8)
Other comprehensive income (loss) attributable to W. R. Grace & Co. shareholders
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	67.9	39.9
Other comprehensive income (loss) before reclassifications	18.3	(23.6)
Amounts reclassified from accumulated other comprehensive income (loss)	(9.7)	7.0
Total other comprehensive income (loss)	8.6	(16.6)
Ending balance	$ 76.5	$ 23.3