Basis of Presentation and Summary of Significant Accounting and Financial Reporting Policies (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other investing activities			$ 13.8	$ (0.1)
Net cash provided by (used for) investing activities			(565.9)	(89.2)
Cash, cash equivalents, and restricted cash, beginning of period	$ 153.3	$ 186.5	153.3	186.5		$ 163.5	$ 100.6
Cash, cash equivalents, and restricted cash, end of period	153.3	186.5	153.3	186.5		163.5	100.6
Net sales	494.9	429.5	1,412.1	1,257.0
Gross profit	202.2	171.3	570.4	491.7
Income (loss) before income taxes	16.6	58.7	148.4	182.7
(Provision for) benefit from income taxes	(0.7)	(11.6)	(50.5)	(49.2)
Net income (loss)	15.9	47.1	97.9	133.5
Net income (loss) attributable to W. R. Grace & Co. Shareholders	16.1	47.4	98.5	134.2
Deferred income taxes	543.6		543.6			556.5
Other liabilities	280.3		280.3			169.3
Retained earnings	625.4		625.4			$ 573.1
Cost of goods sold	292.7	258.2	841.7	765.3
Selling, general and administrative expenses	75.6	69.3	227.1	204.1
Research and development expenses	15.7	14.0	46.5	41.5
Other (income) expense, net	(1.3)	(1.7)	2.2	(15.0)
Accounting Standards Update 2016-18 | Previously Reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other investing activities				(0.9)
Net cash provided by (used for) investing activities				(90.0)
Cash, cash equivalents, and restricted cash, beginning of period		175.7		175.7			90.6
Cash, cash equivalents, and restricted cash, end of period		175.7		175.7			90.6
Accounting Standards Update 2016-18 | Effect of Change
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other investing activities				0.8
Net cash provided by (used for) investing activities				0.8
Cash, cash equivalents, and restricted cash, beginning of period		10.8		10.8			10.0
Cash, cash equivalents, and restricted cash, end of period		10.8		10.8			$ 10.0
Accounting Standards Update 2017-07 | Previously Reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Gross profit		173.3		495.8
Cost of goods sold		256.2		761.2
Selling, general and administrative expenses		70.5		207.3
Research and development expenses		13.3		39.4
Other (income) expense, net		(0.2)		(12.0)
Accounting Standards Update 2017-07 | Revised
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Gross profit		171.3		491.7
Cost of goods sold		258.2		765.3
Selling, general and administrative expenses		70.8		208.6
Research and development expenses		14.0		41.5
Other (income) expense, net		(3.2)		(19.5)
Accounting Standards Update 2017-07 | Effect of Change
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Gross profit		(2.0)		(4.1)
Cost of goods sold		2.0		4.1
Selling, general and administrative expenses		0.3		1.3
Research and development expenses		0.7		2.1
Other (income) expense, net		$ (3.0)		$ (7.5)
Under ASC 605 | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales	494.4		1,411.2
Gross profit	201.7		569.5
Income (loss) before income taxes	16.1		147.5
(Provision for) benefit from income taxes	(0.6)		(50.3)
Net income (loss)	15.5		97.2
Net income (loss) attributable to W. R. Grace & Co. Shareholders	15.7		97.8
Deferred income taxes	544.5		544.5
Other liabilities	284.4		284.4
Retained earnings	622.2		622.2
Effect of Change | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales	0.5		0.9
Gross profit	0.5		0.9
Income (loss) before income taxes	0.5		0.9
(Provision for) benefit from income taxes	(0.1)		(0.2)
Net income (loss)	0.4		0.7
Net income (loss) attributable to W. R. Grace & Co. Shareholders	0.4		0.7
Deferred income taxes	(0.9)		(0.9)		$ 0.7
Other liabilities	(4.1)		(4.1)		3.2
Retained earnings	$ 3.2		$ 3.2		$ 2.5