Guarantee of Registered Securities - Condensed Consolidating Statement of Financial Position (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013	Dec. 31, 2012
CURRENT ASSETS :
Cash and cash equivalents	$ 382	$ 477	$ 369	$ 291
Short-term investments	217	523
Receivables, net	2,630	2,896
Fiduciary assets	10,815	11,871
Intercompany receivables	0	0
Other current assets	711	563
Total Current Assets	14,755	16,330
Goodwill	9,026	8,997
Intangible assets, net	2,612	2,578
Fixed assets, net	760	791
Investments	143	132
Intercompany receivables	0	0
Other non-current assets	1,531	1,423
Investment in subsidiary	0	0
TOTAL ASSETS	28,827	30,251
CURRENT LIABILITIES
Fiduciary liabilities	10,815	11,871
Short-term debt and current portion of long-term debt	660	703
Accounts payable and accrued liabilities	1,528	1,931
Intercompany payables	0	0
Other current liabilities	853	906
Total Current Liabilities	13,856	15,411
Long-term debt	4,840	3,686
Pension, other post-retirement and other post-employment liabilities	1,437	1,607
Intercompany payables	0	0
Other non-current liabilities	1,422	1,352
TOTAL LIABILITIES	21,555	22,056
TOTAL AON SHAREHOLDERS' EQUITY	7,215	8,145
Noncontrolling interests	57	50
TOTAL EQUITY	7,272	8,195
TOTAL LIABILITIES AND EQUITY	28,827	30,251
Aon plc [Member]
CURRENT ASSETS :
Cash and cash equivalents	0	0	118	131
Short-term investments	0	0
Receivables, net	0	0
Fiduciary assets	0	0
Intercompany receivables	321	186
Other current assets	1	0
Total Current Assets	322	186
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Investments	0	0
Intercompany receivables	7,408	7,166
Other non-current assets	169	146
Investment in subsidiary	5,752	4,607
TOTAL ASSETS	13,651	12,105
CURRENT LIABILITIES
Fiduciary liabilities	0	0
Short-term debt and current portion of long-term debt	0	0
Accounts payable and accrued liabilities	1,683	1,036
Intercompany payables	8	15
Other current liabilities	72	12
Total Current Liabilities	1,763	1,063
Long-term debt	2,568	792
Pension, other post-retirement and other post-employment liabilities	0	0
Intercompany payables	2,100	2,100
Other non-current liabilities	5	5
TOTAL LIABILITIES	6,436	3,960
TOTAL AON SHAREHOLDERS' EQUITY	7,215	8,145
Noncontrolling interests	0	0
TOTAL EQUITY	7,215	8,145
TOTAL LIABILITIES AND EQUITY	13,651	12,105
Aon Corporation [Member]
CURRENT ASSETS :
Cash and cash equivalents	568	247	291	199
Short-term investments	59	163
Receivables, net	0	4
Fiduciary assets	0	0
Intercompany receivables	2,696	3,503
Other current assets	205	69
Total Current Assets	3,528	3,986
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Investments	79	57
Intercompany receivables	2,162	2,178
Other non-current assets	498	560
Investment in subsidiary	16,469	11,694
TOTAL ASSETS	22,736	18,475
CURRENT LIABILITIES
Fiduciary liabilities	0	0
Short-term debt and current portion of long-term debt	649	0
Accounts payable and accrued liabilities	34	62
Intercompany payables	9,258	5,449
Other current liabilities	47	47
Total Current Liabilities	9,988	5,558
Long-term debt	1,917	2,512
Pension, other post-retirement and other post-employment liabilities	854	925
Intercompany payables	7,277	7,267
Other non-current liabilities	108	159
TOTAL LIABILITIES	20,144	16,421
TOTAL AON SHAREHOLDERS' EQUITY	2,592	2,054
Noncontrolling interests	0	0
TOTAL EQUITY	2,592	2,054
TOTAL LIABILITIES AND EQUITY	22,736	18,475
Other Non-Guarantor Subsidiaries [Member]
CURRENT ASSETS :
Cash and cash equivalents	1,444	1,246	0	0
Short-term investments	158	360
Receivables, net	2,633	2,892
Fiduciary assets	10,815	11,871
Intercompany receivables	9,419	5,452
Other current assets	669	513
Total Current Assets	25,138	22,334
Goodwill	9,026	8,997
Intangible assets, net	2,612	2,578
Fixed assets, net	760	791
Investments	64	75
Intercompany receivables	2,211	2,201
Other non-current assets	1,550	1,421
Investment in subsidiary	0	0
TOTAL ASSETS	41,361	38,397
CURRENT LIABILITIES
Fiduciary liabilities	10,815	11,871
Short-term debt and current portion of long-term debt	11	707
Accounts payable and accrued liabilities	1,444	1,849
Intercompany payables	3,170	3,677
Other current liabilities	898	866
Total Current Liabilities	16,338	18,970
Long-term debt	355	378
Pension, other post-retirement and other post-employment liabilities	583	682
Intercompany payables	2,404	2,178
Other non-current liabilities	1,995	1,892
TOTAL LIABILITIES	21,675	24,100
TOTAL AON SHAREHOLDERS' EQUITY	19,629	14,247
Noncontrolling interests	57	50
TOTAL EQUITY	19,686	14,297
TOTAL LIABILITIES AND EQUITY	41,361	38,397
Consolidating Adjustments [Member]
CURRENT ASSETS :
Cash and cash equivalents	(1,630)	(1,016)	(40)	(39)
Short-term investments	0	0
Receivables, net	(3)	0
Fiduciary assets	0	0
Intercompany receivables	(12,436)	(9,141)
Other current assets	(164)	(19)
Total Current Assets	(14,233)	(10,176)
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Investments	0	0
Intercompany receivables	(11,781)	(11,545)
Other non-current assets	(686)	(704)
Investment in subsidiary	(22,221)	(16,301)
TOTAL ASSETS	(48,921)	(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	(1,633)	(1,016)
Intercompany payables	(12,436)	(9,141)
Other current liabilities	(164)	(19)
Total Current Liabilities	(14,233)	(10,180)
Long-term debt	0	4
Pension, other post-retirement and other post-employment liabilities	0	0
Intercompany payables	(11,781)	(11,545)
Other non-current liabilities	(686)	(704)
TOTAL LIABILITIES	(26,700)	(22,425)
TOTAL AON SHAREHOLDERS' EQUITY	(22,221)	(16,301)
Noncontrolling interests	0	0
TOTAL EQUITY	(22,221)	(16,301)
TOTAL LIABILITIES AND EQUITY	$ (48,921)	$ (38,726)