Shareholders' Equity - Changes (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019		Sep. 30, 2018
Increase (Decrease) in Stockholders' Equity
Beginning balance		$ 512.3	$ 448.6	$ 481.4		$ 492.0
Net income (loss)		16.7	25.5	40.9		(8.8)
Issuance of common stock in connection with stock plans	[1]			0.0		0.0
Share-based compensation		1.9	(0.5)	7.5		5.3
Exercise of stock options		0.1	0.1	5.1		5.2
Share repurchases		(0.1)		(3.3)	[2]	(1.3)	[2]
Other comprehensive loss		(14.8)	(8.9)	(15.5)		(27.6)
Dividends and other changes in noncontrolling interest			(0.1)			(0.1)
Ending balance		$ 516.1	$ 464.7	$ 516.1		$ 464.7
Share repurchases (in shares)		(5,200)	(56)	(130,900)		(45,000)
Common Stock
Increase (Decrease) in Stockholders' Equity
Beginning balance (in shares)		73,000,000.0	72,400,000	72,400,000		71,900,000
Beginning balance		$ 0.7	$ 0.7	$ 0.7		$ 0.7
Issuance of common stock in connection with stock plans (in shares)	[1]			300,000		100,000
Exercise of stock options (in shares)				300,000		400,000
Ending balance (in shares)		73,000,000.0	72,400,000	73,000,000.0		72,400,000
Ending balance		$ 0.7	$ 0.7	$ 0.7		$ 0.7
Treasury Stock
Increase (Decrease) in Stockholders' Equity
Beginning balance (in shares)		300,000	200,000	200,000		100,000
Beginning balance		$ (8.0)	$ (4.7)	$ (4.8)		$ (3.4)
Share repurchases		$ (0.1)		$ (3.3)	[2]	$ (1.3)	[2]
Ending balance (in shares)		300,000	200,000	300,000		200,000
Ending balance		$ (8.1)	$ (4.7)	$ (8.1)		$ (4.7)
Share repurchases (in shares)	[2]			(100,000)		(100,000)
Additional Paid-in Capital
Increase (Decrease) in Stockholders' Equity
Beginning balance		50.2	40.8	$ 39.6		$ 29.9
Share-based compensation		1.9	(0.5)	7.5		5.3
Exercise of stock options		0.1	0.1	5.1		5.2
Ending balance		52.2	40.4	52.2		40.4
Accumulated Earnings
Increase (Decrease) in Stockholders' Equity
Beginning balance		587.9	514.2	563.9		548.7
Net income (loss)		16.6	25.4	40.6		(9.1)
Ending balance		604.5	539.6	604.5		539.6
Accumulated Other Comprehensive Loss
Increase (Decrease) in Stockholders' Equity
Beginning balance		(120.7)	(104.4)	(120.0)		(85.7)
Other comprehensive loss		(14.8)	(8.9)	(15.5)		(27.6)
Ending balance		(135.5)	(113.3)	(135.5)		(113.3)
Noncontrolling Interests
Increase (Decrease) in Stockholders' Equity
Beginning balance		2.2	2.0	2.0		1.8
Net income (loss)		0.1	0.1	0.3		0.3
Dividends and other changes in noncontrolling interest			(0.1)			(0.1)
Ending balance		$ 2.3	$ 2.0	$ 2.3		$ 2.0

[1] The par value of shares issued is not included in the table due to rounding.

[2] During the nine months ended September 30, 2019 and 2018, GCP withheld and retained approximately 131,000 shares and 45,000 shares, respectively, of Company common stock in a non-cash transaction with a cost of $3.3 million and $1.3 million, respectively, in connection with fulfilling statutory tax withholding requirements for employees under the provisions of the Company's equity compensation programs. During the nine months ended September 30, 2019 and 2018, cash payments for such tax withholding obligations were $3.3 million and $1.3 million, respectively.