Guarantee of Registered Securities (Details 2) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011	Dec. 31, 2010
CURRENT ASSETS :
Cash and cash equivalents	$ 286	$ 272	$ 300	$ 346
Short-term investments	516	785
Receivables, net	3,075	3,183
Fiduciary assets	12,736	10,838
Other current assets	443	427
Total Current Assets	17,056	15,505
Goodwill	8,713	8,770
Intangible assets, net	3,106	3,276
Fixed assets, net	797	783
Investments	200	239
Other non-current assets	935	979
TOTAL ASSETS	30,807	29,552
CURRENT LIABILITIES
Fiduciary liabilities	12,736	10,838
Short-term debt and current portion of long-term debt	392	337
Accounts payable and accrued liabilities	1,339	1,832
Other current liabilities	733	753
Total Current Liabilities	15,200	13,760
Long-term debt	4,098	4,155
Pension and other post employment liabilities	1,967	2,192
Other non-current liabilities	1,332	1,325
TOTAL LIABILITIES	22,597	21,432
TOTAL AON SHAREHOLDERS' EQUITY	8,152	8,078
Noncontrolling interests	58	42
TOTAL EQUITY	8,210	8,120
TOTAL LIABILITIES AND EQUITY	30,807	29,552
Parent Guarantor
CURRENT ASSETS :
Cash and cash equivalents	19
Receivables, net	1
Intercompany receivables	21	3
Total Current Assets	41	3
Investments in subsidiary	10,704	10,183
TOTAL ASSETS	10,745	10,186
CURRENT LIABILITIES
Accounts payable and accrued liabilities	5
Intercompany payables	30
Other current liabilities	1
Total Current Liabilities	36
Intercompany payables	2,400	2,108
Other non-current liabilities	157
TOTAL LIABILITIES	2,593	2,108
TOTAL AON SHAREHOLDERS' EQUITY	8,152	8,078
TOTAL EQUITY	8,152	8,078
TOTAL LIABILITIES AND EQUITY	10,745	10,186
Subsidiary Issuer
CURRENT ASSETS :
Cash and cash equivalents	80	(21)	(5)	14
Short-term investments	111	321
Receivables, net	3	2
Intercompany receivables	1,296	610
Other current assets	90	57
Total Current Assets	1,580	969
Investments	41	39
Intercompany receivables	2,226	2,133
Other non-current assets	704	900
Investments in subsidiary	9,966	9,269
TOTAL ASSETS	14,517	13,310
CURRENT LIABILITIES
Short-term debt and current portion of long-term debt	372	318
Accounts payable and accrued liabilities	75	78
Intercompany payables	1,123	206
Other current liabilities	47	5
Total Current Liabilities	1,617	607
Long-term debt	3,041	3,063
Pension and other post employment liabilities	1,322	1,407
Intercompany payables	360	378
Other non-current liabilities	164	141
TOTAL LIABILITIES	6,504	5,596
TOTAL AON SHAREHOLDERS' EQUITY	8,013	7,714
TOTAL EQUITY	8,013	7,714
TOTAL LIABILITIES AND EQUITY	14,517	13,310
Other non-guarantor subsidiaries
CURRENT ASSETS :
Cash and cash equivalents	187	293	305	332
Short-term investments	405	464
Receivables, net	3,071	3,181
Fiduciary assets	12,736	10,838
Intercompany receivables	1,826	539
Other current assets	353	374
Total Current Assets	18,578	15,689
Goodwill	8,713	8,770
Intangible assets, net	3,106	3,276
Fixed assets, net	797	783
Investments	159	200
Intercompany receivables	2,171	2,202
Other non-current assets	231	79
Investments in subsidiary	8,013	7,714
TOTAL ASSETS	41,768	38,713
CURRENT LIABILITIES
Fiduciary liabilities	12,736	10,838
Short-term debt and current portion of long-term debt	20	19
Accounts payable and accrued liabilities	1,259	1,758
Intercompany payables	1,331	609
Other current liabilities	685	748
Total Current Liabilities	16,031	13,972
Long-term debt	1,057	1,092
Pension and other post employment liabilities	645	785
Intercompany payables	2,296	2,186
Other non-current liabilities	1,011	1,183
TOTAL LIABILITIES	21,040	19,219
TOTAL AON SHAREHOLDERS' EQUITY	20,670	19,452
Noncontrolling interests	58	42
TOTAL EQUITY	20,728	19,494
TOTAL LIABILITIES AND EQUITY	41,768	38,713
Consolidating adjustments
CURRENT ASSETS :
Intercompany receivables	(3,143)	(1,152)
Other current assets		(4)
Total Current Assets	(3,143)	(1,156)
Intercompany receivables	(4,397)	(4,335)
Investments in subsidiary	(28,683)	(27,166)
TOTAL ASSETS	(36,223)	(32,657)
CURRENT LIABILITIES
Accounts payable and accrued liabilities		(4)
Intercompany payables	(2,484)	(815)
Total Current Liabilities	(2,484)	(819)
Intercompany payables	(5,056)	(4,672)
TOTAL LIABILITIES	(7,540)	(5,491)
TOTAL AON SHAREHOLDERS' EQUITY	(28,683)	(27,166)
TOTAL EQUITY	(28,683)	(27,166)
TOTAL LIABILITIES AND EQUITY	$ (36,223)	$ (32,657)