Guarantee of Registered Securities - Condensed Consolidating Statement of Financial Position (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013	Dec. 31, 2012
CURRENT ASSETS :
Cash and cash equivalents	$ 338	$ 477	$ 408	$ 291
Short-term investments	340	523
Receivables, net	2,880	2,896
Fiduciary assets	12,038	11,871
Intercompany receivables	0	0
Other current assets	577	563
Total Current Assets	16,173	16,330
Goodwill	9,030	8,997
Intangible assets, net	2,500	2,578
Fixed assets, net	785	791
Investments	143	132
Intercompany receivables	0	0
Other non-current assets	1,446	1,423
Investment in subsidiary	0	0
TOTAL ASSETS	30,077	30,251
CURRENT LIABILITIES
Fiduciary liabilities	12,038	11,871
Short-term debt and current portion of long-term debt	1,077	703
Accounts payable and accrued liabilities	1,499	1,931
Intercompany payables	0	0
Other current liabilities	967	906
Total Current Liabilities	15,581	15,411
Long-term debt	3,669	3,686
Pension, other post-retirement and other post-employment liabilities	1,573	1,607
Intercompany payables	0	0
Other non-current liabilities	1,248	1,352
TOTAL LIABILITIES	22,071	22,056
TOTAL AON SHAREHOLDERS’ EQUITY	7,946	8,145
Noncontrolling interests	60	50
TOTAL EQUITY	8,006	8,195
TOTAL LIABILITIES AND EQUITY	30,077	30,251
Aon plc [Member]
CURRENT ASSETS :
Cash and cash equivalents	0	0	5	131
Short-term investments	0	0
Receivables, net	0	0
Fiduciary assets	0	0
Intercompany receivables	116	186
Other current assets	0	0
Total Current Assets	116	186
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Investments	0	0
Intercompany receivables	7,166	7,166
Other non-current assets	146	146
Investment in subsidiary	4,979	4,607
TOTAL ASSETS	12,407	12,105
CURRENT LIABILITIES
Fiduciary liabilities	0	0
Short-term debt and current portion of long-term debt	227	0
Accounts payable and accrued liabilities	1,218	1,036
Intercompany payables	87	15
Other current liabilities	33	12
Total Current Liabilities	1,565	1,063
Long-term debt	792	792
Pension, other post-retirement and other post-employment liabilities	0	0
Intercompany payables	2,100	2,100
Other non-current liabilities	4	5
TOTAL LIABILITIES	4,461	3,960
TOTAL AON SHAREHOLDERS’ EQUITY	7,946	8,145
Noncontrolling interests	0	0
TOTAL EQUITY	7,946	8,145
TOTAL LIABILITIES AND EQUITY	12,407	12,105
Aon Corporation [Member]
CURRENT ASSETS :
Cash and cash equivalents	154	247	251	199
Short-term investments	101	163
Receivables, net	1	4
Fiduciary assets	0	0
Intercompany receivables	3,982	3,503
Other current assets	109	69
Total Current Assets	4,347	3,986
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Investments	69	57
Intercompany receivables	2,163	2,178
Other non-current assets	509	560
Investment in subsidiary	12,165	11,694
TOTAL ASSETS	19,253	18,475
CURRENT LIABILITIES
Fiduciary liabilities	0	0
Short-term debt and current portion of long-term debt	140	0
Accounts payable and accrued liabilities	41	62
Intercompany payables	5,834	5,449
Other current liabilities	47	47
Total Current Liabilities	6,062	5,558
Long-term debt	2,516	2,512
Pension, other post-retirement and other post-employment liabilities	930	925
Intercompany payables	7,267	7,267
Other non-current liabilities	112	159
TOTAL LIABILITIES	16,887	16,421
TOTAL AON SHAREHOLDERS’ EQUITY	2,366	2,054
Noncontrolling interests	0	0
TOTAL EQUITY	2,366	2,054
TOTAL LIABILITIES AND EQUITY	19,253	18,475
Other Non-Guarantor Subsidiaries [Member]
CURRENT ASSETS :
Cash and cash equivalents	1,372	1,246	152	0
Short-term investments	239	360
Receivables, net	2,879	2,892
Fiduciary assets	12,038	11,871
Intercompany receivables	5,878	5,452
Other current assets	538	513
Total Current Assets	22,944	22,334
Goodwill	9,030	8,997
Intangible assets, net	2,500	2,578
Fixed assets, net	785	791
Investments	74	75
Intercompany receivables	2,201	2,201
Other non-current assets	1,477	1,421
Investment in subsidiary	0	0
TOTAL ASSETS	39,011	38,397
CURRENT LIABILITIES
Fiduciary liabilities	12,038	11,871
Short-term debt and current portion of long-term debt	710	707
Accounts payable and accrued liabilities	1,428	1,849
Intercompany payables	4,055	3,677
Other current liabilities	957	866
Total Current Liabilities	19,188	18,970
Long-term debt	361	378
Pension, other post-retirement and other post-employment liabilities	643	682
Intercompany payables	2,163	2,178
Other non-current liabilities	1,818	1,892
TOTAL LIABILITIES	24,173	24,100
TOTAL AON SHAREHOLDERS’ EQUITY	14,778	14,247
Noncontrolling interests	60	50
TOTAL EQUITY	14,838	14,297
TOTAL LIABILITIES AND EQUITY	39,011	38,397
Consolidating Adjustments [Member]
CURRENT ASSETS :
Cash and cash equivalents	(1,188)	(1,016)	0	(39)
Short-term investments	0	0
Receivables, net	0	0
Fiduciary assets	0	0
Intercompany receivables	(9,976)	(9,141)
Other current assets	(70)	(19)
Total Current Assets	(11,234)	(10,176)
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Investments	0	0
Intercompany receivables	(11,530)	(11,545)
Other non-current assets	(686)	(704)
Investment in subsidiary	(17,144)	(16,301)
TOTAL ASSETS	(40,594)	(38,726)
CURRENT LIABILITIES
Fiduciary liabilities	0	0
Short-term debt and current portion of long-term debt	0	(4)
Accounts payable and accrued liabilities	(1,188)	(1,016)
Intercompany payables	(9,976)	(9,141)
Other current liabilities	(70)	(19)
Total Current Liabilities	(11,234)	(10,180)
Long-term debt	0	4
Pension, other post-retirement and other post-employment liabilities	0	0
Intercompany payables	(11,530)	(11,545)
Other non-current liabilities	(686)	(704)
TOTAL LIABILITIES	(23,450)	(22,425)
TOTAL AON SHAREHOLDERS’ EQUITY	(17,144)	(16,301)
Noncontrolling interests	0	0
TOTAL EQUITY	(17,144)	(16,301)
TOTAL LIABILITIES AND EQUITY	$ (40,594)	$ (38,726)