Other Comprehensive Income - Pre-Tax, Tax, and After-Tax Components of Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
After-Tax Amount
Total other comprehensive income	$ 3.2	$ 14.3
Other comprehensive income attributable to GCP shareholders
Pre-Tax Amount
Other comprehensive (loss) income, pre-tax amount	3.2	14.3
Tax Benefit/ (Expense)
Other comprehensive (loss) income, tax	0.0	0.0
After-Tax Amount
Total other comprehensive income	3.2	14.3
Currency translation adjustments
Pre-Tax Amount
Other comprehensive (loss) income, pre-tax amount	3.3	14.3
Tax Benefit/ (Expense)
Other comprehensive (loss) income, tax	0.0	0.0
After-Tax Amount
Total other comprehensive income	3.3	14.3
Loss from hedging activities
Pre-Tax Amount
Other comprehensive (loss) income, pre-tax amount	(0.1)
Tax Benefit/ (Expense)
Other comprehensive (loss) income, tax	0.0
After-Tax Amount
Total other comprehensive income	$ (0.1)	$ 0.0