Other Comprehensive Loss - OCI Components (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2017		Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pre-Tax Amount	$ (0.6)		$ (0.6)	$ (1.8)	$ (1.9)
Tax Benefit/ (Expense)	0.2		0.2	0.6	0.7
After-Tax Amount	(0.4)		(0.4)	(1.2)	(1.2)
Defined Benefit Pension and Other Postretirement Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pre-Tax Amount	0.1		0.1	0.3	0.4
Tax Benefit/ (Expense)	0.0		0.0	(0.1)	(0.2)
After-Tax Amount	0.1		0.1	0.2	0.2
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pre-Tax Amount	(0.5)		(0.5)	(1.5)	(1.5)
Tax Benefit/ (Expense)	0.2		0.2	0.5	0.5
After-Tax Amount	(0.3)		(0.3)	(1.0)	(1.0)
Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pre-Tax Amount	(3.6)		(2.3)	(17.9)	(6.4)
Tax Benefit/ (Expense)	(8.5)	[1]	0.0	(3.9)	0.0
After-Tax Amount	(12.1)		(2.3)	(21.8)	(6.4)
Gain (Loss) from Hedging Activities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pre-Tax Amount	0.7		0.9	1.5	(4.2)
Tax Benefit/ (Expense)	(1.1)		(0.3)	(1.4)	1.5
After-Tax Amount	(0.4)		0.6	0.1	(2.7)
AOCI Attributable to Parent
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pre-Tax Amount	(3.4)		(1.9)	(17.9)	(12.1)
Tax Benefit/ (Expense)	(9.4)		(0.1)	(4.8)	2.0
After-Tax Amount	$ (12.8)		$ (2.0)	$ (22.7)	$ (10.1)

[1] (1) Tax expense relates to Grace's euro loan net investment hedge, and includes an out-of-period adjustment to recognize the accumulated deferred tax liability related to this hedge. See Note 5.