Guarantee of Registered Securities - Condensed Colsolidating Statement of Financial Position (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012	Dec. 31, 2011
CURRENT ASSETS :
Cash and cash equivalents	$ 266	$ 291	$ 286	$ 272
Short-term investments	307	346
Receivables, net	2,838	3,101
Fiduciary assets	12,576	12,214
Intercompany receivables	0	0
Other current assets	406	430
Total Current Assets	16,393	16,382
Goodwill	8,795	8,943
Intangible assets, net	2,736	2,975
Fixed assets, net	808	820
Investments	172	165
Intercompany receivables	0	0
Other non-current assets	1,158	1,201
Investment in subsidiary	0	0
TOTAL ASSETS	30,062	30,486
CURRENT LIABILITIES
Fiduciary liabilities	12,576	12,214
Short-term debt and current portion of long-term debt	427	452
Accounts payable and accrued liabilities	1,423	1,853
Intercompany payables	0	0
Other current liabilities	769	831
Total Current Liabilities	15,195	15,350
Long-term debt	4,012	3,713
Pension, other post-retirement and other post-employment liabilities	1,897	2,276
Intercompany payables	0	0
Other non-current liabilities	1,340	1,342
TOTAL LIABILITIES	22,444	22,681
TOTAL AON SHAREHOLDERS’ EQUITY	7,559	7,762
Noncontrolling interests	59	43
TOTAL EQUITY	7,618	7,805
TOTAL LIABILITIES AND EQUITY	30,062	30,486
Aon plc [Member]
CURRENT ASSETS :
Cash and cash equivalents	294	131	19	0
Short-term investments	0	0
Receivables, net	0	5
Fiduciary assets	0	0
Intercompany receivables	255	0
Other current assets	17	7
Total Current Assets	566	143
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Investments	0	0
Intercompany receivables	166	166
Other non-current assets	119	117
Investment in subsidiary	10,879	10,398
TOTAL ASSETS	11,730	10,824
CURRENT LIABILITIES
Fiduciary liabilities	0	0
Short-term debt and current portion of long-term debt	297	0
Accounts payable and accrued liabilities	17	10
Intercompany payables	14	51
Other current liabilities	1	0
Total Current Liabilities	329	61
Long-term debt	443	107
Pension, other post-retirement and other post-employment liabilities	0	0
Intercompany payables	3,393	2,890
Other non-current liabilities	6	4
TOTAL LIABILITIES	4,171	3,062
TOTAL AON SHAREHOLDERS’ EQUITY	7,559	7,762
Noncontrolling interests	0	0
TOTAL EQUITY	7,559	7,762
TOTAL LIABILITIES AND EQUITY	11,730	10,824
Aon Corporation [Member]
CURRENT ASSETS :
Cash and cash equivalents	258	199	80	(21)
Short-term investments	63	89
Receivables, net	(2)	1
Fiduciary assets	0	0
Intercompany receivables	2,881	2,092
Other current assets	50	53
Total Current Assets	3,250	2,434
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Investments	60	49
Intercompany receivables	2,093	1,997
Other non-current assets	739	735
Investment in subsidiary	11,057	10,522
TOTAL ASSETS	17,199	15,737
CURRENT LIABILITIES
Fiduciary liabilities	0	0
Short-term debt and current portion of long-term debt	107	429
Accounts payable and accrued liabilities	65	71
Intercompany payables	4,112	2,637
Other current liabilities	47	49
Total Current Liabilities	4,331	3,186
Long-term debt	2,515	2,515
Pension, other post-retirement and other post-employment liabilities	1,220	1,294
Intercompany payables	166	166
Other non-current liabilities	251	254
TOTAL LIABILITIES	8,483	7,415
TOTAL AON SHAREHOLDERS’ EQUITY	8,716	8,322
Noncontrolling interests	0	0
TOTAL EQUITY	8,716	8,322
TOTAL LIABILITIES AND EQUITY	17,199	15,737
Other Non-Guarantor Subsidiaries [Member]
CURRENT ASSETS :
Cash and cash equivalents	0	0	187	293
Short-term investments	244	257
Receivables, net	2,840	3,095
Fiduciary assets	12,576	12,214
Intercompany receivables	5,244	3,545
Other current assets	339	370
Total Current Assets	21,243	19,481
Goodwill	8,795	8,943
Intangible assets, net	2,736	2,975
Fixed assets, net	808	820
Investments	112	116
Intercompany receivables	2,196	2,350
Other non-current assets	403	1,174
Investment in subsidiary	8,716	8,322
TOTAL ASSETS	45,009	44,181
CURRENT LIABILITIES
Fiduciary liabilities	12,576	12,214
Short-term debt and current portion of long-term debt	23	23
Accounts payable and accrued liabilities	1,627	1,811
Intercompany payables	2,961	2,162
Other current liabilities	721	779
Total Current Liabilities	17,908	16,989
Long-term debt	1,054	1,091
Pension, other post-retirement and other post-employment liabilities	677	982
Intercompany payables	2,189	2,247
Other non-current liabilities	1,186	1,909
TOTAL LIABILITIES	23,014	23,218
TOTAL AON SHAREHOLDERS’ EQUITY	21,936	20,920
Noncontrolling interests	59	43
TOTAL EQUITY	21,995	20,963
TOTAL LIABILITIES AND EQUITY	45,009	44,181
Consolidating Adjustments [Member]
CURRENT ASSETS :
Cash and cash equivalents	(286)	(39)	0	0
Short-term investments	0	0
Receivables, net	0	0
Fiduciary assets	0	0
Intercompany receivables	(8,380)	(5,637)
Other current assets	0	0
Total Current Assets	(8,666)	(5,676)
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Investments	0	0
Intercompany receivables	(4,455)	(4,513)
Other non-current assets	(103)	(825)
Investment in subsidiary	(30,652)	(29,242)
TOTAL ASSETS	(43,876)	(40,256)
CURRENT LIABILITIES
Fiduciary liabilities	0	0
Short-term debt and current portion of long-term debt	0	0
Accounts payable and accrued liabilities	(286)	(39)
Intercompany payables	(7,087)	(4,850)
Other current liabilities	0	3
Total Current Liabilities	(7,373)	(4,886)
Long-term debt	0	0
Pension, other post-retirement and other post-employment liabilities	0	0
Intercompany payables	(5,748)	(5,303)
Other non-current liabilities	(103)	(825)
TOTAL LIABILITIES	(13,224)	(11,014)
TOTAL AON SHAREHOLDERS’ EQUITY	(30,652)	(29,242)
Noncontrolling interests	0	0
TOTAL EQUITY	(30,652)	(29,242)
TOTAL LIABILITIES AND EQUITY	(43,876)	(40,256)
Pro Forma [Member]
CURRENT ASSETS :
Investment in subsidiary	0	0
CURRENT LIABILITIES
TOTAL AON SHAREHOLDERS’ EQUITY	7,559	7,762
Pro Forma [Member] | Aon plc [Member]
CURRENT ASSETS :
Investment in subsidiary	10,879	10,398
CURRENT LIABILITIES
TOTAL AON SHAREHOLDERS’ EQUITY	7,559	7,762
Pro Forma [Member] | Aon Corporation [Member]
CURRENT ASSETS :
Investment in subsidiary	11,057	10,522
CURRENT LIABILITIES
TOTAL AON SHAREHOLDERS’ EQUITY	8,716	8,322
Pro Forma [Member] | Other Non-Guarantor Subsidiaries [Member]
CURRENT ASSETS :
Investment in subsidiary	0	0
CURRENT LIABILITIES
TOTAL AON SHAREHOLDERS’ EQUITY	13,220	12,598
Pro Forma [Member] | Consolidating Adjustments [Member]
CURRENT ASSETS :
Investment in subsidiary	(21,936)	(20,920)
CURRENT LIABILITIES
TOTAL AON SHAREHOLDERS’ EQUITY	$ (21,936)	$ (20,920)