Guarantee of Registered Securities - Condensed Consolidating Statement of Financial Position (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013	Dec. 31, 2012
CURRENT ASSETS :
Cash and cash equivalents	$ 418	$ 477	$ 266	$ 291
Short-term investments	308	523
Receivables, net	2,803	2,896
Fiduciary assets	12,786	11,871
Intercompany receivables	0	0
Other current assets	1,396	563
Total Current Assets	17,711	16,330
Goodwill	9,099	8,997
Intangible assets, net	2,468	2,578
Fixed assets, net	787	791
Investments	138	132
Intercompany receivables	0	0
Other non-current assets	1,573	1,423
Investment in subsidiary	0	0
TOTAL ASSETS	31,776	30,251
CURRENT LIABILITIES
Fiduciary liabilities	12,786	11,871
Short-term debt and current portion of long-term debt	799	703
Accounts payable and accrued liabilities	1,498	1,931
Intercompany payables	0	0
Other current liabilities	957	906
Total Current Liabilities	16,040	15,411
Long-term debt	5,155	3,686
Pension, other post-retirement and other post-employment liabilities	1,533	1,607
Intercompany payables	0	0
Other non-current liabilities	1,289	1,352
TOTAL LIABILITIES	24,017	22,056
TOTAL AON SHAREHOLDERS’ EQUITY	7,700	8,145
Noncontrolling interests	59	50
TOTAL EQUITY	7,759	8,195
TOTAL LIABILITIES AND EQUITY	31,776	30,251
Aon plc [Member]
CURRENT ASSETS :
Cash and cash equivalents	0	0	294	131
Short-term investments	0	0
Receivables, net	0	0
Fiduciary assets	0	0
Intercompany receivables	224	186
Other current assets	1	0
Total Current Assets	225	186
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Investments	0	0
Intercompany receivables	7,426	7,166
Other non-current assets	151	146
Investment in subsidiary	5,634	4,607
TOTAL ASSETS	13,436	12,105
CURRENT LIABILITIES
Fiduciary liabilities	0	0
Short-term debt and current portion of long-term debt	107	0
Accounts payable and accrued liabilities	1,214	1,036
Intercompany payables	2	15
Other current liabilities	42	12
Total Current Liabilities	1,365	1,063
Long-term debt	2,267	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	5,736	3,960
TOTAL AON SHAREHOLDERS’ EQUITY	7,700	8,145
Noncontrolling interests	0	0
TOTAL EQUITY	7,700	8,145
TOTAL LIABILITIES AND EQUITY	13,436	12,105
Aon Corporation [Member]
CURRENT ASSETS :
Cash and cash equivalents	181	247	258	199
Short-term investments	136	163
Receivables, net	1	4
Fiduciary assets	0	0
Intercompany receivables	4,261	3,503
Other current assets	165	69
Total Current Assets	4,744	3,986
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Investments	73	57
Intercompany receivables	2,177	2,178
Other non-current assets	501	560
Investment in subsidiary	12,673	11,694
TOTAL ASSETS	20,168	18,475
CURRENT LIABILITIES
Fiduciary liabilities	0	0
Short-term debt and current portion of long-term debt	0	0
Accounts payable and accrued liabilities	54	62
Intercompany payables	6,586	5,449
Other current liabilities	52	47
Total Current Liabilities	6,692	5,558
Long-term debt	2,508	2,512
Pension, other post-retirement and other post-employment liabilities	895	925
Intercompany payables	7,277	7,267
Other non-current liabilities	108	159
TOTAL LIABILITIES	17,480	16,421
TOTAL AON SHAREHOLDERS’ EQUITY	2,688	2,054
Noncontrolling interests	0	0
TOTAL EQUITY	2,688	2,054
TOTAL LIABILITIES AND EQUITY	20,168	18,475
Other Non-Guarantor Subsidiaries [Member]
CURRENT ASSETS :
Cash and cash equivalents	1,416	1,246	0	0
Short-term investments	172	360
Receivables, net	2,802	2,892
Fiduciary assets	12,786	11,871
Intercompany receivables	6,668	5,452
Other current assets	1,342	513
Total Current Assets	25,186	22,334
Goodwill	9,099	8,997
Intangible assets, net	2,468	2,578
Fixed assets, net	787	791
Investments	65	75
Intercompany receivables	2,211	2,201
Other non-current assets	1,607	1,421
Investment in subsidiary	0	0
TOTAL ASSETS	41,423	38,397
CURRENT LIABILITIES
Fiduciary liabilities	12,786	11,871
Short-term debt and current portion of long-term debt	701	707
Accounts payable and accrued liabilities	1,409	1,849
Intercompany payables	4,565	3,677
Other current liabilities	975	866
Total Current Liabilities	20,436	18,970
Long-term debt	371	378
Pension, other post-retirement and other post-employment liabilities	638	682
Intercompany payables	2,437	2,178
Other non-current liabilities	1,863	1,892
TOTAL LIABILITIES	25,745	24,100
TOTAL AON SHAREHOLDERS’ EQUITY	15,619	14,247
Noncontrolling interests	59	50
TOTAL EQUITY	15,678	14,297
TOTAL LIABILITIES AND EQUITY	41,423	38,397
Consolidating Adjustments [Member]
CURRENT ASSETS :
Cash and cash equivalents	(1,179)	(1,016)	(286)	(39)
Short-term investments	0	0
Receivables, net	0	0
Fiduciary assets	0	0
Intercompany receivables	(11,153)	(9,141)
Other current assets	(112)	(19)
Total Current Assets	(12,444)	(10,176)
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Investments	0	0
Intercompany receivables	(11,814)	(11,545)
Other non-current assets	(686)	(704)
Investment in subsidiary	(18,307)	(16,301)
TOTAL ASSETS	(43,251)	(38,726)
CURRENT LIABILITIES
Fiduciary liabilities	0	0
Short-term debt and current portion of long-term debt	(9)	(4)
Accounts payable and accrued liabilities	(1,179)	(1,016)
Intercompany payables	(11,153)	(9,141)
Other current liabilities	(112)	(19)
Total Current Liabilities	(12,453)	(10,180)
Long-term debt	9	4
Pension, other post-retirement and other post-employment liabilities	0	0
Intercompany payables	(11,814)	(11,545)
Other non-current liabilities	(686)	(704)
TOTAL LIABILITIES	(24,944)	(22,425)
TOTAL AON SHAREHOLDERS’ EQUITY	(18,307)	(16,301)
Noncontrolling interests	0	0
TOTAL EQUITY	(18,307)	(16,301)
TOTAL LIABILITIES AND EQUITY	$ (43,251)	$ (38,726)