Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Numerators
Income from continuing operations attributable to GCP shareholders		$ 1.4	$ 14.6
(Loss) income from discontinued operations, net of income taxes		(0.3)	6.8
Net income attributable to GCP shareholders		$ 1.1	$ 21.4
Denominators
Weighted average common shares—basic calculation (in shares)		72.9	72.3
Dilutive effect of employee stock awards (in shares)		0.1	0.5
Weighted average common shares—diluted calculation (in shares)	[1]	73.0	72.8
Basic earnings (loss) per share:
Income from continuing operations attributable to GCP shareholders (in usd per share)		$ 0.02	$ 0.20
(Loss) income from discontinued operations, net of income taxes (in usd per share)		0	0.09
Net income attributable to GCP shareholders (in usd per share)	[2]	0.02	0.30
Diluted earnings (loss) per share
Income from continuing operations attributable to GCP shareholders (in dollars per share)	[1]	0.02	0.20
(Loss) income from discontinued operations, net of income taxes (in dollars per share)	[1]	0	0.09
Net income attributable to GCP shareholders (in dollars per share)	[1],[2]	$ 0.02	$ 0.29
Antidilutive securities excluded from computation of earnings per share, treasury stock method (in shares)		0.6	0.6

[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.