Other Comprehensive Income (Loss) - Pre-Tax, Tax, and After-Tax Components of Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
After-Tax Amount
Total other comprehensive income (loss)	$ 3.9	$ (29.4)
Net prior service credit (cost)
Pre-Tax Amount
Amortization of net prior service cost included in net periodic benefit cost, pre-tax	(0.1)	0.1
Assumption of net prior service credit and net actuarial loss, pre-tax	1.2
Tax Benefit/ (Expense)
Amortization of net prior service cost included in net periodic benefit cost, tax	0.0	(0.1)
Assumption of net prior service credit and net actuarial loss, tax	(0.4)
After-Tax Amount
Amortization of net prior service cost included in net periodic benefit cost, after-tax	(0.1)	0.0
Assumption of net prior service credit and net actuarial loss, after-tax	0.8
Net actuarial gain (loss)
Pre-Tax Amount
Assumption of net prior service credit and net actuarial loss, pre-tax	(1.1)
Tax Benefit/ (Expense)
Assumption of net prior service credit and net actuarial loss, tax	0.4
After-Tax Amount
Assumption of net prior service credit and net actuarial loss, after-tax	(0.7)
Benefit plans, net
Pre-Tax Amount
Other comprehensive income (loss), pre-tax	0.0	0.1
Tax Benefit/ (Expense)
Other comprehensive income (loss), tax	0.0	(0.1)
After-Tax Amount
Amortization of net prior service cost included in net periodic benefit cost, after-tax	0.1	0.0
Assumption of net prior service credit and net actuarial loss, after-tax	0.1	0.0
Total other comprehensive income (loss)	0.0	0.0
Currency translation adjustments
Pre-Tax Amount
Other comprehensive income (loss), pre-tax	3.9	(29.6)
Tax Benefit/ (Expense)
Other comprehensive income (loss), tax	0.0	0.0
After-Tax Amount
Amortization of net prior service cost included in net periodic benefit cost, after-tax	0.0	0.0
Assumption of net prior service credit and net actuarial loss, after-tax	3.9	(29.6)
Total other comprehensive income (loss)	3.9	(29.6)
Gain (loss) from hedging activities
Pre-Tax Amount
Other comprehensive income (loss), pre-tax	(0.2)	0.2
Tax Benefit/ (Expense)
Other comprehensive income (loss), tax	0.0	(0.1)
After-Tax Amount
Amortization of net prior service cost included in net periodic benefit cost, after-tax	(0.3)	(0.2)
Assumption of net prior service credit and net actuarial loss, after-tax	(0.5)	(0.1)
Total other comprehensive income (loss)	(0.2)	0.1
Other comprehensive income attributable to GCP shareholders
Pre-Tax Amount
Other comprehensive income (loss), pre-tax	3.7	(29.3)
Tax Benefit/ (Expense)
Other comprehensive income (loss), tax	0.0	(0.2)
After-Tax Amount
Amortization of net prior service cost included in net periodic benefit cost, after-tax	(0.2)	(0.2)
Assumption of net prior service credit and net actuarial loss, after-tax	3.5	(29.7)
Total other comprehensive income (loss)	$ 3.7	$ (29.5)