Stock Incentive Plans - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended				12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019		Sep. 30, 2018		Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock compensation (benefit) expense	$ 2.3	$ (0.3)	$ 7.3		$ 4.6
Share repurchases (in shares)	5,200	56	130,900		45,000
Share repurchases	$ 0.1		$ 3.3	[1]	$ 1.3	[1]
Cash payments for tax withholding obligations			$ 3.3		1.3
Shares of common stock reserved and available for future grant (in shares)	7,700,000		7,700,000
Weighted average grant date fair value of options granted (in usd per share)	$ 6.66	$ 8.57
Fair value of awards vested			$ 6.4		4.8
Performance Based Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period			3 years
Performance Based Units | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected payout, percent of target			0.00%
Performance Based Units | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected payout, percent of target			200.00%
Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Intrinsic value of all options exercised		$ 0.1	$ 2.5		$ 4.8
Unrecognized stock-based compensation expense for stock options	$ 1.2		$ 1.2
Unrecognized stock-based compensation expense for stock options, period of recognition			1 year 2 months 12 days
Restricted Stock Units (RSUs) and Performance Based Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized stock-based compensation expense for stock options, period of recognition			1 year 8 months 12 days
Total unrecognized compensation expense related to the RSU and PBU awards	$ 5.7		$ 5.7
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period			3 years
Weighted average grant date fair value (in usd per share)	$ 21.52	$ 25.20	$ 26.77		$ 29.28
Amount settled in period (in shares)			255,000		115,932
Cash settlement of awards					$ 1.2
Awards granted (in shares)			137,000
Awards forfeited (in shares)			27,000
Awards outstanding (in shares)	218,000	400,000	218,000		400,000		363,000
Awards outstanding, weighted average grant date fair value (in usd per share)	$ 27.49		$ 27.49				$ 22.76
Restricted Stock Units (RSUs) | Vesting 1, Year 1
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting percentage			33.33%
Restricted Stock Units (RSUs) | Vesting 1, Year 2
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting percentage			33.33%
Restricted Stock Units (RSUs) | Vesting 1, Year 3
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting percentage			33.33%
Restricted Stock Units (RSUs) | Vesting 2, Year 1
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting percentage			0.00%
Restricted Stock Units (RSUs) | Vesting 2, Year 2
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting percentage			0.00%
Restricted Stock Units (RSUs) | Vesting 2, Year 3
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting percentage			100.00%
Sign-On Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period			2 years
Award vesting percentage			100.00%
2016 Grant Date | Performance Based Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Amount settled in period (in shares)			76,461
Performance period			3 years
Percentage of target award granted			68.70%
Awards forfeited (in shares)			34,840
Awards vested - weighted average granted date fair value (in usd per share)			$ 17.04
2016 Grant Date | Performance Based Units | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected payout, percent of target			0.00%
2016 Grant Date | Performance Based Units | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected payout, percent of target			200.00%
2017 and 2018 Grant Dates | Performance Based Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance period			3 years
2019 Grant Date | Performance Based Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant date fair value (in usd per share)			$ 27.19
Awards granted (in shares)			174,302
Awards forfeited (in shares)			6,180
Awards outstanding (in shares)	168,122		168,122
Awards outstanding, weighted average grant date fair value (in usd per share)	$ 27.20		$ 27.20
2018 Grant Date | Performance Based Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant date fair value (in usd per share)							$ 34.14
Awards granted (in shares)							149,974
Awards forfeited (in shares)			17,844
Awards outstanding (in shares)	125,953		125,953				143,797
Awards outstanding, weighted average grant date fair value (in usd per share)	$ 34.06		$ 34.06
2017 Grant Date | Performance Based Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock compensation (benefit) expense		$ (2.8)
Awards granted (in shares)								166,821
Awards forfeited (in shares)			14,479
Awards outstanding (in shares)	124,436		124,436				138,915
Awards outstanding, weighted average grant date fair value (in usd per share)	$ 28.22		$ 28.22				$ 28.25

[1] 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.