Financial Information for Parent Guarantor, Other Guarantor Subsidiaries and Non-Guarantor Subsidiaries (Details 2) (USD $) In Millions	6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010
Condensed Consolidated Statements of Cash Flows [Abstract]
NET CASH PROVIDED BY (USED IN) OPERATING ACTIVITIES	$ 126	$ 154	[1]
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds on disposal of fixed and intangible assets	5	4	[1]
Additions to fixed assets	(47)	(45)	[1]
Acquisitions of subsidiaries, net of cash acquired	(4)	(15)	[1]
Acquisition of investments in associates	(2)	(1)	[1]
Investment in Trident V Parallel Fund, LP	(4)
Net cash used in investing activities	(52)	(57)	[1]
CASH FLOWS FROM FINANCING ACTIVITIES
(Repayment) proceeds from draw down of revolving credit facility	(90)	30	[1]
Senior notes issued	794
Debt issuance costs	(7)
Repayments of debt	(555)	(70)	[1]
Make-whole on repurchase and redemption of senior notes	(158)
Proceeds from issue of shares	42	17	[1]
Excess tax benefits from share-based payment arrangement	4	1	[1]
Dividends paid	(90)	(89)	[1]
Acquisition of noncontrolling interests	(9)	(4)	[1]
Dividends paid to noncontrolling interests	(12)	(22)	[1]
Net cash used in financing activities	(81)	(137)	[1]
INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	(7)	(40)	[1]
Effect of exchange rate changes on cash and cash equivalents	8	(14)	[1]
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	316	221	[1]
CASH AND CASH EQUIVALENTS, END OF PERIOD	317	167	[1]
Willis Group Holdings
Condensed Consolidated Statements of Cash Flows [Abstract]
NET CASH PROVIDED BY (USED IN) OPERATING ACTIVITIES	3	569
CASH FLOWS FROM FINANCING ACTIVITIES
Senior notes issued	794
Debt issuance costs	(7)
Proceeds from issue of shares	42	17
Amounts owed by and to Group undertakings	(742)	(542)
Dividends paid	(90)	(44)
Net cash used in financing activities	(3)	(569)
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	0	0
CASH AND CASH EQUIVALENTS, END OF PERIOD	0	0
The Other Guarantors [Member]
Condensed Consolidated Statements of Cash Flows [Abstract]
NET CASH PROVIDED BY (USED IN) OPERATING ACTIVITIES	76	321
CASH FLOWS FROM FINANCING ACTIVITIES
Repayments of debt	(500)
Make-whole on repurchase and redemption of senior notes	(158)
Amounts owed by and to Group undertakings	590	(189)
Dividends paid		(132)
Acquisition of noncontrolling interests	(8)
Net cash used in financing activities	(76)	(321)
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	0	0
CASH AND CASH EQUIVALENTS, END OF PERIOD	0	0
The Issuer [Member]
Condensed Consolidated Statements of Cash Flows [Abstract]
NET CASH PROVIDED BY (USED IN) OPERATING ACTIVITIES	54	42
CASH FLOWS FROM INVESTING ACTIVITIES
Additions to fixed assets	(13)	(13)
Net cash used in investing activities	(13)	(13)
CASH FLOWS FROM FINANCING ACTIVITIES
(Repayment) proceeds from draw down of revolving credit facility	(90)	30
Repayments of debt	(55)	(61)
Amounts owed by and to Group undertakings	115	(21)
Net cash used in financing activities	(30)	(52)
INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	11	(23)
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	76	104
CASH AND CASH EQUIVALENTS, END OF PERIOD	87	81
Other [Member]
Condensed Consolidated Statements of Cash Flows [Abstract]
NET CASH PROVIDED BY (USED IN) OPERATING ACTIVITIES	140	(22)
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds on disposal of fixed and intangible assets	5	4
Additions to fixed assets	(34)	(32)
Acquisitions of subsidiaries, net of cash acquired	(4)	(15)
Acquisition of investments in associates	(2)	(1)
Investment in Trident V Parallel Fund, LP	(4)
Net cash used in investing activities	(39)	(44)
CASH FLOWS FROM FINANCING ACTIVITIES
Repayments of debt		(9)
Amounts owed by and to Group undertakings	37	752
Excess tax benefits from share-based payment arrangement	4	1
Dividends paid	(147)	(669)
Acquisition of noncontrolling interests	(1)	(4)
Dividends paid to noncontrolling interests	(12)	(22)
Net cash used in financing activities	(119)	49
INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	(18)	(17)
Effect of exchange rate changes on cash and cash equivalents	8	(14)
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	240	117
CASH AND CASH EQUIVALENTS, END OF PERIOD	230	86
Consolidating adjustments [Member]
Condensed Consolidated Statements of Cash Flows [Abstract]
NET CASH PROVIDED BY (USED IN) OPERATING ACTIVITIES	(147)	(756)
CASH FLOWS FROM FINANCING ACTIVITIES
Dividends paid	147	756
Net cash used in financing activities	147	756
CASH AND CASH EQUIVALENTS, BEGINNING OF PERIOD	0	0
CASH AND CASH EQUIVALENTS, END OF PERIOD	$ 0	$ 0

[1] The 2010 Unaudited Condensed Consolidated Statement of Cash Flows has been recast to conform to the new balance sheet presentation. See Note 2 - Basis of Presentation and Significant Accounting Policies for details.