Guarantee of Registered Securities (Details 2) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
CURRENT ASSETS :
Cash and cash equivalents	$ 291	$ 272	$ 346	$ 217
Short-term investments	346	785
Receivables, net	3,101	3,183
Fiduciary assets	12,214	10,838
Other current assets	430	427
Total Current Assets	16,382	15,505
Goodwill	8,943	8,770	8,647
Intangible assets, net	2,975	3,276
Fixed assets, net	820	783
Investments	165	239
Deferred tax assets	285	258
Other non-current assets	916	721
TOTAL ASSETS	30,486	29,552
CURRENT LIABILITIES
Fiduciary liabilities	12,214	10,838
Short-term debt and current portion of long-term debt	452	337
Accounts payable and accrued liabilities	1,853	1,832
Other current liabilities	831	753
Total Current Liabilities	15,350	13,760
Long-term debt	3,713	4,155
Deferred tax liabilities	306	301
Pension and other post employment liabilities	2,276	2,192
Other non-current liabilities	1,036	1,024
TOTAL LIABILITIES	22,681	21,432
TOTAL AON SHAREHOLDERS' EQUITY	7,762	8,078
Noncontrolling interests	43	42
TOTAL EQUITY	7,805	8,120	8,306	5,431
TOTAL LIABILITIES AND EQUITY	30,486	29,552
Aon plc
CURRENT ASSETS :
Cash and cash equivalents	131
Receivables, net	5
Intercompany receivables		3
Other current assets	7
Total Current Assets	143	3
Deferred tax assets	117
Intercompany receivables	166
Investments in subsidiary	10,398	10,183
TOTAL ASSETS	10,824	10,186
CURRENT LIABILITIES
Accounts payable and accrued liabilities	10
Intercompany payables	51
Total Current Liabilities	61
Long-term debt	107
Intercompany payables	2,890	2,108
Other non-current liabilities	4
TOTAL LIABILITIES	3,062	2,108
TOTAL AON SHAREHOLDERS' EQUITY	7,762	8,078
TOTAL EQUITY	7,762	8,078
TOTAL LIABILITIES AND EQUITY	10,824	10,186
Aon Corporation
CURRENT ASSETS :
Cash and cash equivalents	199	(21)	14	(5)
Short-term investments	89	321
Receivables, net	1	2
Intercompany receivables	2,092	610
Other current assets	53	57
Total Current Assets	2,434	969
Investments	49	39
Deferred tax assets	620	702
Intercompany receivables	1,997	2,133
Other non-current assets	115	198
Investments in subsidiary	10,522	9,269
TOTAL ASSETS	15,737	13,310
CURRENT LIABILITIES
Short-term debt and current portion of long-term debt	429	318
Accounts payable and accrued liabilities	71	78
Intercompany payables	2,637	206
Other current liabilities	49	5
Total Current Liabilities	3,186	607
Long-term debt	2,515	3,063
Pension and other post employment liabilities	1,294	1,407
Intercompany payables	166	378
Other non-current liabilities	254	141
TOTAL LIABILITIES	7,415	5,596
TOTAL AON SHAREHOLDERS' EQUITY	8,322	7,714
TOTAL EQUITY	8,322	7,714
TOTAL LIABILITIES AND EQUITY	15,737	13,310
Other Non-Guarantor Subsidiaries
CURRENT ASSETS :
Cash and cash equivalents	(39)	293	332	222
Short-term investments	257	464
Receivables, net	3,095	3,181
Fiduciary assets	12,214	10,838
Intercompany receivables	3,545	539
Other current assets	370	378
Total Current Assets	19,442	15,693
Goodwill	8,943	8,770
Intangible assets, net	2,975	3,276
Fixed assets, net	820	783
Investments	116	200
Deferred tax assets	266	258
Intercompany receivables	2,350	2,202
Other non-current assets	908	523
Investments in subsidiary	8,322	7,714
TOTAL ASSETS	44,142	39,419
CURRENT LIABILITIES
Fiduciary liabilities	12,214	10,838
Short-term debt and current portion of long-term debt	23	19
Accounts payable and accrued liabilities	1,772	1,758
Intercompany payables	2,162	609
Other current liabilities	779	752
Total Current Liabilities	16,950	13,976
Long-term debt	1,091	1,092
Deferred tax liabilities	1,024	1,003
Pension and other post employment liabilities	982	785
Intercompany payables	2,247	2,186
Other non-current liabilities	885	883
TOTAL LIABILITIES	23,179	19,925
TOTAL AON SHAREHOLDERS' EQUITY	20,920	19,452
Noncontrolling interests	43	42
TOTAL EQUITY	20,963	19,494
TOTAL LIABILITIES AND EQUITY	44,142	39,419
Consolidating Adjustments
CURRENT ASSETS :
Intercompany receivables	(5,637)	(1,152)
Other current assets		(8)
Total Current Assets	(5,637)	(1,160)
Deferred tax assets	(718)	(702)
Intercompany receivables	(4,513)	(4,335)
Other non-current assets	(107)
Investments in subsidiary	(29,242)	(27,166)
TOTAL ASSETS	(40,217)	(33,363)
CURRENT LIABILITIES
Accounts payable and accrued liabilities		(4)
Intercompany payables	(4,850)	(815)
Other current liabilities	3	(4)
Total Current Liabilities	(4,847)	(823)
Deferred tax liabilities	(718)	(702)
Intercompany payables	(5,303)	(4,672)
Other non-current liabilities	(107)
TOTAL LIABILITIES	(10,975)	(6,197)
TOTAL AON SHAREHOLDERS' EQUITY	(29,242)	(27,166)
TOTAL EQUITY	(29,242)	(27,166)
TOTAL LIABILITIES AND EQUITY	$ (40,217)	$ (33,363)