Earnings Per Share - Earnings Per Share Reconciliation (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Numerators
Income (loss) from continuing operations attributable to GCP shareholders		$ 17.0	$ 7.2	$ 34.7	$ (35.8)
(Loss) income from discontinued operations, net of income taxes		(0.4)	18.2	5.9	26.7
Net income (loss) attributable to GCP shareholders		$ 16.6	$ 25.4	$ 40.6	$ (9.1)
Denominators
Weighted average common shares—basic calculation		72.7	72.2	72.5	72.1
Dilutive effect of employee stock awards (in shares)		0.1	0.5	0.4	0.0
Weighted average common shares—diluted calculation	[1]	72.8	72.7	72.9	72.1
Basic earnings (loss) per share
Income (loss) from continuing operations attributable to GCP shareholders (in usd per share)		$ 0.23	$ 0.10	$ 0.48	$ (0.50)
(Loss) income from discontinued operations, net of income taxes (in usd per share)		(0.01)	0.25	0.08	0.37
Net income (loss) attributable to GCP shareholders (in usd per share)	[2]	0.23	0.35	0.56	(0.13)
Diluted earnings (loss) per share
Income (loss) from continuing operations attributable to GCP shareholders (in dollars per share)	[1]	0.23	0.10	0.48	(0.50)
(Loss) income from discontinued operations, net of income taxes (in dollars per share)	[1]	(0.01)	0.25	0.08	0.37
Net income (loss) attributable to GCP shareholders (in dollars per share)	[1],[2]	$ 0.23	$ 0.35	$ 0.56	$ (0.13)

[1] Dilutive effect is only applicable to the periods during which GCP generated net income from continuing operations.

[2] Amounts may not sum due to rounding.