Equity and Noncontrolling Interests (Details) (USD $) In Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Equity and Noncontrolling Interests
Balance at beginning of period			$ 2,577
Balance at beginning of period, noncontrolling interest			31
Balance at beginning of period, total			2,608	2,229
Comprehensive income
Net Income (Loss) Attributable to Parent	85	89	119	293
Net income	89	91	131	302	[1]
Other comprehensive income, net of tax	21	(16)	53	(16)
Comprehensive income attributable to parent	106	73	172	277
Comprehensive income attributable noncontrolling interest	4	2	12	9
Comprehensive income, total	110	75	184	286
Dividends			(102)	(111)
Additional paid-in capital			48	33
Purchase of subsidiary shares from noncontrolling interests			0	(4)
Foreign currency translation			1	(4)
Balance at end of period	2,707		2,707
Balance at end of period, noncontrolling interest	32		32
Balance at end of period, total	2,739	2,429	2,739	2,429
Willis Group Holdings stockholders [Member]
Equity and Noncontrolling Interests
Balance at beginning of period			2,577	2,180
Comprehensive income
Net Income (Loss) Attributable to Parent			119	293
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent			53	(16)
Comprehensive income attributable to parent			172	277
Dividends			(90)	(89)
Additional paid-in capital			48	33
Purchase of subsidiary shares from noncontrolling interests			0
Foreign currency translation, attributable to parent			0
Balance at end of period	2,707	2,401	2,707	2,401
Noncontrolling Interest [Member]
Equity and Noncontrolling Interests
Balance at beginning of period, noncontrolling interest			31	49
Comprehensive income
Net income			12	9
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest			0
Comprehensive income attributable noncontrolling interest			12	9
Dividends			(12)	(22)
Additional paid-in capital			0
Purchase of subsidiary shares from noncontrolling interests			0	(4)
Foreign currency translation, portion attributable to noncontrolling interest			1	(4)
Balance at end of period, noncontrolling interest	$ 32	$ 28	$ 32	$ 28

[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.