Guarantee of Registered Securities - Condensed Consolidating Statement of Financial Position (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
CURRENT ASSETS :
Cash and cash equivalents	$ 378	$ 374	$ 338	$ 477
Short-term investments	343	394
Receivables, net	2,672	2,815
Fiduciary assets	10,894	11,638
Intercompany receivables	0	0
Other current assets	712	602
Total Current Assets	14,999	15,823
Goodwill	8,548	8,860
Intangible assets, net	2,410	2,520
Fixed assets, net	754	765
Intercompany receivables	0	0
Other non-current assets	1,678	1,804
Investment in subsidiary	0	0
TOTAL ASSETS	28,389	29,772
CURRENT LIABILITIES
Fiduciary liabilities	10,894	11,638
Short-term debt and current portion of long-term debt	966	783
Accounts payable and accrued liabilities	1,391	1,805
Intercompany payables	0	0
Other current liabilities	865	788
Total Current Liabilities	14,116	15,014
Long-term debt	4,710	4,799
Pension, other post-retirement and post-employment liabilities	2,001	2,141
Intercompany payables	0	0
Other non-current liabilities	1,131	1,187
TOTAL LIABILITIES	21,958	23,141
TOTAL AON SHAREHOLDERS' EQUITY	6,365	6,571
Noncontrolling interests	66	60
TOTAL EQUITY	6,431	6,631
TOTAL LIABILITIES AND EQUITY	28,389	29,772
Aon plc [Member]
CURRENT ASSETS :
Cash and cash equivalents	0	0	0	0
Short-term investments	0	0
Receivables, net	0	0
Fiduciary assets	0	0
Intercompany receivables	125	455
Other current assets	2	2
Total Current Assets	127	457
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Intercompany receivables	7,374	7,399
Other non-current assets	181	179
Investment in subsidiary	4,121	4,962
TOTAL ASSETS	11,803	12,997
CURRENT LIABILITIES
Fiduciary liabilities	0	0
Short-term debt and current portion of long-term debt	109	0
Accounts payable and accrued liabilities	2,597	3,755
Intercompany payables	246	122
Other current liabilities	0	0
Total Current Liabilities	2,952	3,877
Long-term debt	2,480	2,544
Pension, other post-retirement and post-employment liabilities	0	0
Intercompany payables	0	0
Other non-current liabilities	6	5
TOTAL LIABILITIES	5,438	6,426
TOTAL AON SHAREHOLDERS' EQUITY	6,365	6,571
Noncontrolling interests	0	0
TOTAL EQUITY	6,365	6,571
TOTAL LIABILITIES AND EQUITY	11,803	12,997
Aon Corporation [Member]
CURRENT ASSETS :
Cash and cash equivalents	2,230	2,727	154	247
Short-term investments	166	165
Receivables, net	0	0
Fiduciary assets	0	0
Intercompany receivables	3,581	2,814
Other current assets	207	226
Total Current Assets	6,184	5,932
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Intercompany receivables	577	600
Other non-current assets	701	697
Investment in subsidiary	15,391	15,200
TOTAL ASSETS	22,853	22,429
CURRENT LIABILITIES
Fiduciary liabilities	0	0
Short-term debt and current portion of long-term debt	846	767
Accounts payable and accrued liabilities	31	58
Intercompany payables	9,382	8,960
Other current liabilities	49	49
Total Current Liabilities	10,308	9,834
Long-term debt	1,917	1,917
Pension, other post-retirement and post-employment liabilities	1,366	1,396
Intercompany payables	7,287	7,277
Other non-current liabilities	107	125
TOTAL LIABILITIES	20,985	20,549
TOTAL AON SHAREHOLDERS' EQUITY	1,868	1,880
Noncontrolling interests	0	0
TOTAL EQUITY	1,868	1,880
TOTAL LIABILITIES AND EQUITY	22,853	22,429
Other Non-Guarantor Subsidiaries [Member]
CURRENT ASSETS :
Cash and cash equivalents	650	1,361	1,372	1,246
Short-term investments	177	229
Receivables, net	2,672	2,815
Fiduciary assets	10,894	11,638
Intercompany receivables	9,681	9,156
Other current assets	536	407
Total Current Assets	24,610	25,606
Goodwill	8,548	8,860
Intangible assets, net	2,410	2,520
Fixed assets, net	754	765
Intercompany receivables	121	111
Other non-current assets	1,589	1,718
Investment in subsidiary	1,868	1,880
TOTAL ASSETS	39,900	41,460
CURRENT LIABILITIES
Fiduciary liabilities	10,894	11,638
Short-term debt and current portion of long-term debt	11	16
Accounts payable and accrued liabilities	1,265	1,706
Intercompany payables	3,759	3,343
Other current liabilities	849	772
Total Current Liabilities	16,778	17,475
Long-term debt	313	338
Pension, other post-retirement and post-employment liabilities	635	745
Intercompany payables	785	833
Other non-current liabilities	1,811	1,847
TOTAL LIABILITIES	20,322	21,238
TOTAL AON SHAREHOLDERS' EQUITY	19,512	20,162
Noncontrolling interests	66	60
TOTAL EQUITY	19,578	20,222
TOTAL LIABILITIES AND EQUITY	39,900	41,460
Consolidating Adjustments [Member]
CURRENT ASSETS :
Cash and cash equivalents	(2,502)	(3,714)	(1,188)	(1,016)
Short-term investments	0	0
Receivables, net	0	0
Fiduciary assets	0	0
Intercompany receivables	(13,387)	(12,425)
Other current assets	(33)	(33)
Total Current Assets	(15,922)	(16,172)
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Intercompany receivables	(8,072)	(8,110)
Other non-current assets	(793)	(790)
Investment in subsidiary	(21,380)	(22,042)
TOTAL ASSETS	(46,167)	(47,114)
CURRENT LIABILITIES
Fiduciary liabilities	0	0
Short-term debt and current portion of long-term debt	0	0
Accounts payable and accrued liabilities	(2,502)	(3,714)
Intercompany payables	(13,387)	(12,425)
Other current liabilities	(33)	(33)
Total Current Liabilities	(15,922)	(16,172)
Long-term debt	0	0
Pension, other post-retirement and post-employment liabilities	0	0
Intercompany payables	(8,072)	(8,110)
Other non-current liabilities	(793)	(790)
TOTAL LIABILITIES	(24,787)	(25,072)
TOTAL AON SHAREHOLDERS' EQUITY	(21,380)	(22,042)
Noncontrolling interests	0	0
TOTAL EQUITY	(21,380)	(22,042)
TOTAL LIABILITIES AND EQUITY	$ (46,167)	$ (47,114)