Earnings Per Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2013	Mar. 31, 2012
Numerators
Net income attributable to W. R. Grace and Co. shareholders	$ 52.9	$ 60.9
Denominators
Weighted average common shares-basic calculation	75.7	74.3
Dilutive effect of employee stock options (in shares)	1.5	2.1
Weighted average common shares-diluted calculation	77.2	76.4
Basic earnings per share (in dollars per share)	$ 0.70	$ 0.82
Diluted earnings per share (in dollars per share)	$ 0.69	$ 0.80
Number of warrants not included in diluted earnings per share	10

The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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+ Details

- Definition

The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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- Definition

The average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period.

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- Definition

Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period.

+ References

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-Glossary Weighted-Average Number of Common Shares Outstanding

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+ Details