Consolidated Statements of Comprehensive (Loss) Income (unaudited) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Statement of Comprehensive Income [Abstract]
Net (loss) income	$ (1.2)	$ 2.6	$ 0.0	$ 24.2
Other comprehensive income (loss):
Defined benefit pension and other postretirement plans, net of income taxes	0.0	0.0	0.1	0.0
Currency translation adjustments, net of income taxes	4.9	(3.9)	(28.9)	(0.6)
(Loss) gain from hedging activities, net of income taxes	(0.1)	0.0	0.1	(0.1)
After-Tax Amount	4.8	(3.9)	(28.7)	(0.7)
Comprehensive income (loss)	3.6	(1.3)	(28.7)	23.5
Less: Comprehensive income attributable to noncontrolling interests	(0.1)	0.0	(0.2)	(0.2)
Comprehensive income (loss) attributable to GCP shareholders	$ 3.5	$ (1.3)	$ (28.9)	$ 23.3