Guarantee of Registered Securities (Details 2) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2011	Dec. 31, 2010
CURRENT ASSETS :
Cash and cash equivalents	$ 290	$ 272	$ 295	$ 346
Short-term investments	718	785
Receivables, net	2,959	3,183
Fiduciary assets	11,343	10,838
Other current assets	392	427
Total Current Assets	15,702	15,505
Goodwill	8,856	8,770
Intangible assets, net	3,040	3,276
Fixed assets, net	814	783
Investments	160	239
Other non-current assets	937	979
TOTAL ASSETS	29,509	29,552
CURRENT LIABILITIES
Fiduciary liabilities	11,343	10,838
Short-term debt and current portion of long-term debt	291	337
Accounts payable and accrued liabilities	1,519	1,832
Other current liabilities	801	753
Total Current Liabilities	13,954	13,760
Long-term debt	4,111	4,155
Pension, other post retirement and other post employment liabilities	1,757	2,192
Other non-current liabilities	1,326	1,325
TOTAL LIABILITIES	21,148	21,432
TOTAL AON SHAREHOLDERS' EQUITY	8,305	8,078
Noncontrolling interests	56	42
TOTAL EQUITY	8,361	8,120
TOTAL LIABILITIES AND EQUITY	29,509	29,552
Parent Guarantor
CURRENT ASSETS :
Cash and cash equivalents	1
Short-term investments	16
Intercompany receivables		3
Total Current Assets	17	3
Other non-current assets	222
Investments in subsidiary	10,797	10,183
TOTAL ASSETS	11,036	10,186
CURRENT LIABILITIES
Accounts payable and accrued liabilities	4
Intercompany payables	34
Total Current Liabilities	38
Intercompany payables	2,690	2,108
Other non-current liabilities	3
TOTAL LIABILITIES	2,731	2,108
TOTAL AON SHAREHOLDERS' EQUITY	8,305	8,078
TOTAL EQUITY	8,305	8,078
TOTAL LIABILITIES AND EQUITY	11,036	10,186
Subsidiary Issuer
CURRENT ASSETS :
Cash and cash equivalents	58	(21)	(26)	14
Short-term investments	261	321
Receivables, net	2	2
Intercompany receivables	1,677	610
Other current assets	41	57
Total Current Assets	2,039	969
Investments	41	39
Intercompany receivables	2,240	2,133
Other non-current assets	659	900
Investments in subsidiary	10,349	9,269
TOTAL ASSETS	15,328	13,310
CURRENT LIABILITIES
Short-term debt and current portion of long-term debt	268	318
Accounts payable and accrued liabilities	86	78
Intercompany payables	1,779	206
Other current liabilities	47	5
Total Current Liabilities	2,180	607
Long-term debt	3,030	3,063
Pension, other post retirement and other post employment liabilities	1,167	1,407
Intercompany payables	359	378
Other non-current liabilities	146	141
TOTAL LIABILITIES	6,882	5,596
TOTAL AON SHAREHOLDERS' EQUITY	8,446	7,714
TOTAL EQUITY	8,446	7,714
TOTAL LIABILITIES AND EQUITY	15,328	13,310
Other non-guarantor subsidiaries
CURRENT ASSETS :
Cash and cash equivalents	231	293	321	332
Short-term investments	441	464
Receivables, net	2,957	3,181
Fiduciary assets	11,343	10,838
Intercompany receivables	2,868	539
Other current assets	351	378
Total Current Assets	18,191	15,693
Goodwill	8,856	8,770
Intangible assets, net	3,040	3,276
Fixed assets, net	814	783
Investments	119	200
Intercompany receivables	2,171	2,202
Other non-current assets	56	781
Investments in subsidiary	8,446	7,714
TOTAL ASSETS	41,693	39,419
CURRENT LIABILITIES
Fiduciary liabilities	11,343	10,838
Short-term debt and current portion of long-term debt	23	19
Accounts payable and accrued liabilities	1,429	1,758
Intercompany payables	1,782	609
Other current liabilities	754	752
Total Current Liabilities	15,331	13,976
Long-term debt	1,081	1,092
Pension, other post retirement and other post employment liabilities	590	785
Intercompany payables	2,311	2,186
Other non-current liabilities	1,177	1,886
TOTAL LIABILITIES	20,490	19,925
TOTAL AON SHAREHOLDERS' EQUITY	21,146	19,452
Noncontrolling interests	56	42
TOTAL EQUITY	21,202	19,494
TOTAL LIABILITIES AND EQUITY	41,692	39,419
Consolidating adjustments
CURRENT ASSETS :
Intercompany receivables	(4,545)	(1,152)
Other current assets		(8)
Total Current Assets	(4,545)	(1,160)
Intercompany receivables	(4,411)	(4,335)
Other non-current assets		(702)
Investments in subsidiary	(29,592)	(27,166)
TOTAL ASSETS	(38,548)	(33,363)
CURRENT LIABILITIES
Accounts payable and accrued liabilities		(4)
Intercompany payables	(3,595)	(815)
Other current liabilities		(4)
Total Current Liabilities	(3,595)	(823)
Intercompany payables	(5,360)	(4,672)
Other non-current liabilities		(702)
TOTAL LIABILITIES	(8,955)	(6,197)
TOTAL AON SHAREHOLDERS' EQUITY	(29,592)	(27,166)
TOTAL EQUITY	(29,592)	(27,166)
TOTAL LIABILITIES AND EQUITY	$ (38,547)	$ (33,363)