Basis of Presentation and Summary of Significant Accounting and Financial Reporting Policies (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other investing activities			$ 12.7	$ 0.3
Net cash provided by (used for) investing activities			(499.0)	(58.8)
Cash, cash equivalents, and restricted cash, beginning of period	$ 132.8	$ 172.6	132.8	172.6		$ 163.5	$ 100.6
Cash, cash equivalents, and restricted cash, end of period	132.8	172.6	132.8	172.6		163.5	100.6
Net sales	485.7	429.5	917.2	827.5
Gross profit	198.7	167.2	368.2	320.4
Income (loss) before income taxes	63.6	63.1	131.8	124.0
(Provision for) benefit from income taxes	(25.0)	(19.6)	(49.8)	(37.6)
Net income (loss)	38.6	43.5	82.0	86.4
Net income (loss) attributable to W. R. Grace & Co. Shareholders	38.8	43.9	82.4	86.8
Deferred income taxes	535.4		535.4			556.5
Other liabilities	188.7		188.7			169.3
Retained earnings	625.5		625.5			$ 573.1
Cost of goods sold	287.0	262.3	549.0	507.1
Selling, general and administrative expenses	82.2	69.3	151.5	134.8
Research and development expenses	16.1	13.6	30.8	27.5
Other (income) expense, net	5.8	(11.4)	3.5	(13.3)
Previously Reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Gross profit		169.3		322.5
Cost of goods sold		260.2		505.0
Selling, general and administrative expenses		70.3		136.8
Research and development expenses		12.9		26.1
Other (income) expense, net		(9.6)		(11.8)
Accounting Standards Update 2016-18
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other investing activities				0.3
Net cash provided by (used for) investing activities				(58.8)
Cash, cash equivalents, and restricted cash, beginning of period		172.6		172.6			100.6
Cash, cash equivalents, and restricted cash, end of period		172.6		172.6			100.6
Accounting Standards Update 2016-18 | Previously Reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other investing activities				(0.5)
Net cash provided by (used for) investing activities				(59.6)
Cash, cash equivalents, and restricted cash, beginning of period		161.8		161.8			90.6
Cash, cash equivalents, and restricted cash, end of period		161.8		161.8			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 | Revised
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Gross profit		167.2		320.4
Cost of goods sold		262.3		507.1
Selling, general and administrative expenses		70.8		137.8
Research and development expenses		13.6		27.5
Other (income) expense, net		(12.9)		(16.3)
Accounting Standards Update 2017-07 | Effect of Change
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Gross profit		(2.1)		(2.1)
Cost of goods sold		2.1		2.1
Selling, general and administrative expenses		0.5		1.0
Research and development expenses		0.7		1.4
Other (income) expense, net		$ (3.3)		$ (4.5)
Under ASC 605
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales	485.4		916.8
Gross profit	198.4		367.8
Income (loss) before income taxes	63.3		131.4
(Provision for) benefit from income taxes	(24.9)		(49.7)
Net income (loss)	38.4		81.7
Net income (loss) attributable to W. R. Grace & Co. Shareholders	38.6		82.1
Deferred income taxes	536.2		536.2
Other liabilities	192.3		192.3
Retained earnings	622.7		622.7
Effect of Change | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net sales	0.3		0.4
Gross profit	0.3		0.4
Income (loss) before income taxes	0.3		0.4
(Provision for) benefit from income taxes	(0.1)		(0.1)
Net income (loss)	0.2		0.3
Net income (loss) attributable to W. R. Grace & Co. Shareholders	0.2		0.3
Deferred income taxes	(0.8)		(0.8)		$ (0.7)
Other liabilities	(3.6)		(3.6)		(3.2)
Retained earnings	$ 2.8		$ 2.8		$ 2.5