Guarantee of Registered Securities - Condensed Consolidating Statement of Financial Position (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Dec. 31, 2011
CURRENT ASSETS :
Cash and cash equivalents	$ 369	$ 291	$ 290	$ 272
Short-term investments	357	346
Receivables, net	2,666	3,101
Fiduciary assets	11,698	12,214
Intercompany receivables	0	0
Other current assets	484	430
Total Current Assets	15,574	16,382
Goodwill	8,958	8,943
Intangible assets, net	2,657	2,975
Fixed assets, net	803	820
Investments	139	165
Intercompany receivables	0	0
Other non-current assets	1,174	1,201
Investment in subsidiary	0	0
TOTAL ASSETS	29,305	30,486
CURRENT LIABILITIES
Fiduciary liabilities	11,698	12,214
Short-term debt and current portion of long-term debt	1,206	452
Accounts payable and accrued liabilities	1,523	1,853
Intercompany payables	0	0
Other current liabilities	764	831
Total Current Liabilities	15,191	15,350
Long-term debt	3,352	3,713
Pension, other post-retirement and other post-employment liabilities	1,773	2,276
Intercompany payables	0	0
Other non-current liabilities	1,399	1,342
TOTAL LIABILITIES	21,715	22,681
TOTAL AON SHAREHOLDERS’ EQUITY	7,534	7,762
Noncontrolling interests	56	43
TOTAL EQUITY	7,590	7,805
TOTAL LIABILITIES AND EQUITY	29,305	30,486
Aon plc [Member]
CURRENT ASSETS :
Cash and cash equivalents	118	131	1	0
Short-term investments	0	0
Receivables, net	0	5
Fiduciary assets	0	0
Intercompany receivables	41	0
Other current assets	19	7
Total Current Assets	178	143
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Investments	0	0
Intercompany receivables	7,166	166
Other non-current assets	119	117
Investment in subsidiary	4,306	10,398
TOTAL ASSETS	11,769	10,824
CURRENT LIABILITIES
Fiduciary liabilities	0	0
Short-term debt and current portion of long-term debt	361	0
Accounts payable and accrued liabilities	16	10
Intercompany payables	1,308	51
Other current liabilities	1	0
Total Current Liabilities	1,686	61
Long-term debt	443	107
Pension, other post-retirement and other post-employment liabilities	0	0
Intercompany payables	2,100	2,890
Other non-current liabilities	6	4
TOTAL LIABILITIES	4,235	3,062
TOTAL AON SHAREHOLDERS’ EQUITY	7,534	7,762
Noncontrolling interests	0	0
TOTAL EQUITY	7,534	7,762
TOTAL LIABILITIES AND EQUITY	11,769	10,824
Aon Corporation [Member]
CURRENT ASSETS :
Cash and cash equivalents	291	199	58	(21)
Short-term investments	124	89
Receivables, net	5	1
Fiduciary assets	0	0
Intercompany receivables	3,250	2,092
Other current assets	59	53
Total Current Assets	3,729	2,434
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Investments	53	49
Intercompany receivables	2,181	1,997
Other non-current assets	748	735
Investment in subsidiary	11,187	10,208
TOTAL ASSETS	17,898	15,423
CURRENT LIABILITIES
Fiduciary liabilities	0	0
Short-term debt and current portion of long-term debt	148	429
Accounts payable and accrued liabilities	41	71
Intercompany payables	4,727	2,637
Other current liabilities	47	49
Total Current Liabilities	4,963	3,186
Long-term debt	2,516	2,515
Pension, other post-retirement and other post-employment liabilities	1,162	1,294
Intercompany payables	7,262	166
Other non-current liabilities	260	254
TOTAL LIABILITIES	16,163	7,415
TOTAL AON SHAREHOLDERS’ EQUITY	1,735	8,008
Noncontrolling interests	0	0
TOTAL EQUITY	1,735	8,008
TOTAL LIABILITIES AND EQUITY	17,898	15,423
Other Non-Guarantor Subsidiaries [Member]
CURRENT ASSETS :
Cash and cash equivalents	0	0	231	293
Short-term investments	233	257
Receivables, net	2,661	3,095
Fiduciary assets	11,698	12,214
Intercompany receivables	6,176	3,545
Other current assets	406	370
Total Current Assets	21,174	19,481
Goodwill	8,958	8,943
Intangible assets, net	2,657	2,975
Fixed assets, net	803	820
Investments	86	116
Intercompany receivables	2,195	2,350
Other non-current assets	408	1,174
Investment in subsidiary	0	0
TOTAL ASSETS	36,281	35,859
CURRENT LIABILITIES
Fiduciary liabilities	11,698	12,214
Short-term debt and current portion of long-term debt	697	23
Accounts payable and accrued liabilities	1,506	1,811
Intercompany payables	3,430	2,162
Other current liabilities	718	779
Total Current Liabilities	18,049	16,989
Long-term debt	393	1,091
Pension, other post-retirement and other post-employment liabilities	611	982
Intercompany payables	2,180	2,247
Other non-current liabilities	1,234	1,909
TOTAL LIABILITIES	22,467	23,218
TOTAL AON SHAREHOLDERS’ EQUITY	13,758	12,598
Noncontrolling interests	56	43
TOTAL EQUITY	13,814	12,641
TOTAL LIABILITIES AND EQUITY	36,281	35,859
Consolidating Adjustments [Member]
CURRENT ASSETS :
Cash and cash equivalents	(40)	(39)	0	0
Short-term investments	0	0
Receivables, net	0	0
Fiduciary assets	0	0
Intercompany receivables	(9,467)	(5,637)
Other current assets	0	0
Total Current Assets	(9,507)	(5,676)
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Investments	0	0
Intercompany receivables	(11,542)	(4,513)
Other non-current assets	(101)	(825)
Investment in subsidiary	(15,493)	(20,606)
TOTAL ASSETS	(36,643)	(31,620)
CURRENT LIABILITIES
Fiduciary liabilities	0	0
Short-term debt and current portion of long-term debt	0	0
Accounts payable and accrued liabilities	(40)	(39)
Intercompany payables	(9,465)	(4,850)
Other current liabilities	(2)	3
Total Current Liabilities	(9,507)	(4,886)
Long-term debt	0	0
Pension, other post-retirement and other post-employment liabilities	0	0
Intercompany payables	(11,542)	(5,303)
Other non-current liabilities	(101)	(825)
TOTAL LIABILITIES	(21,150)	(11,014)
TOTAL AON SHAREHOLDERS’ EQUITY	(15,493)	(20,606)
Noncontrolling interests	0	0
TOTAL EQUITY	(15,493)	(20,606)
TOTAL LIABILITIES AND EQUITY	$ (36,643)	$ (31,620)