Earnings Per Share (Details) (USD $) In Millions, except Per Share data	3 Months Ended	6 Months Ended
Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Basic and diluted earnings per share
Net income attributable to Willis Group Holdings	$ 85	$ 89	$ 119	$ 293
Basic weighted average number of shares outstanding	172	170	172	169
Dilutive effect of potentially issuable shares	4	1	3	2
Diluted weighted average number of shares outstanding	176	171	175	171
Basic earnings per share:
Net income attributable to Willis Group Holdings shareholders, Basic	$ 0.49	$ 0.52	$ 0.69	$ 1.73
Dilutive effect of potentially issuable shares	$ (0.01)		$ (0.01)	$ (0.02)
Diluted earnings per share:
Net income attributable to Willis Group Holdings shareholders, Diluted	$ 0.48	$ 0.52	$ 0.68	$ 1.71

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

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The amount of net income or loss for the period per each share of common stock and dilutive common stock equivalents outstanding during the reporting period.

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The portion of consolidated profit or loss for the period, net of income taxes, which is attributable to the parent. If the entity does not present consolidated financial statements, the amount of profit or loss for the period, net of income taxes.

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The sum of dilutive potential common shares used in the calculation of the diluted per-share computation.

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The average number of shares issued and outstanding that are used in calculating diluted EPS, determined based on the timing of issuance of shares in the period.

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

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Basic and diluted earnings per share.

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