Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended			6 Months Ended
		Jun. 30, 2020	Jun. 30, 2019		Jun. 30, 2020		Jun. 30, 2019
Numerators
(Loss) income from continuing operations attributable to GCP shareholders		$ (1.3)	$ 3.1		$ 0.1		$ 17.7
(Loss) income from discontinued operations, net of income taxes		0.0	(0.5)		(0.3)		6.3
Net (loss) income attributable to GCP shareholders		$ (1.3)	$ 2.6		$ (0.2)		$ 24.0
Denominators
Weighted average common shares—basic calculation (in shares)	[1]	72,900,000	72,600,000		72,900,000		72,500,000
Dilutive effect of employee stock awards (in shares)		0	400,000		100,000		400,000
Weighted average common shares—diluted calculation (in shares)		72,900,000	73,000,000.0	[1]	73,000,000.0	[1]	72,900,000	[1]
Basic earnings (loss) per share:
(Loss) income from continuing operations attributable to GCP shareholders (in usd per share)	[1]	$ (0.02)	$ 0.04		$ 0		$ 0.24
(Loss) income from discontinued operations, net of income taxes (in usd per share)	[1]	0	(0.01)		0		0.09
Net (loss) income attributable to GCP shareholders (in usd per share)	[1],[2]	(0.02)	0.04		0		0.33
Diluted earnings (loss) per share
(Loss) income from continuing operations attributable to GCP shareholders (in dollars per share)	[1]	(0.02)	0.04		0		0.24
(Loss) income from discontinued operations, net of income taxes (in dollars per share)	[1]	0	(0.01)		0		0.09
Net (loss) income attributable to GCP shareholders (in dollars per share)	[1],[2]	$ (0.02)	$ 0.04		$ 0		$ 0.33
Antidilutive securities excluded from computation of earnings per share, treasury stock method (in shares)		0	600,000		800,000		600,000

[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.