Guarantee of Registered Securities - Condensed Consolidating Statement of Financial Position (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013
CURRENT ASSETS :
Cash and cash equivalents	$ 398	$ 374	$ 382	$ 477
Short-term investments	385	394
Receivables, net	2,453	2,815
Fiduciary assets	9,956	11,638
Intercompany receivables	0	0
Other current assets	728	602
Total Current Assets	13,920	15,823
Goodwill	8,571	8,860
Intangible assets, net	2,257	2,520
Fixed assets, net	785	765
Intercompany receivables	0	0
Other non-current assets	1,673	1,804
Investment in subsidiary	0	0
TOTAL ASSETS	27,206	29,772
CURRENT LIABILITIES
Fiduciary liabilities	9,956	11,638
Short-term debt and current portion of long-term debt	1,327	783
Accounts payable and accrued liabilities	1,447	1,805
Intercompany payables	0	0
Other current liabilities	811	788
Total Current Liabilities	13,541	15,014
Long-term debt	4,799	4,799
Pension, other post-retirement and post-employment liabilities	1,833	2,141
Intercompany payables	0	0
Other non-current liabilities	1,091	1,187
TOTAL LIABILITIES	21,264	23,141
TOTAL AON SHAREHOLDERS' EQUITY	5,883	6,571
Noncontrolling interests	59	60
TOTAL EQUITY	5,942	6,631
TOTAL LIABILITIES AND EQUITY	27,206	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	360	455
Other current assets	2	2
Total Current Assets	362	457
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Intercompany receivables	7,380	7,399
Other non-current assets	187	179
Investment in subsidiary	4,627	4,962
TOTAL ASSETS	12,556	12,997
CURRENT LIABILITIES
Fiduciary liabilities	0	0
Short-term debt and current portion of long-term debt	56	0
Accounts payable and accrued liabilities	3,240	3,755
Intercompany payables	277	122
Other current liabilities	0	0
Total Current Liabilities	3,573	3,877
Long-term debt	3,094	2,544
Pension, other post-retirement and post-employment liabilities	0	0
Intercompany payables	0	0
Other non-current liabilities	6	5
TOTAL LIABILITIES	6,673	6,426
TOTAL AON SHAREHOLDERS' EQUITY	5,883	6,571
Noncontrolling interests	0	0
TOTAL EQUITY	5,883	6,571
TOTAL LIABILITIES AND EQUITY	12,556	12,997
Aon Corporation [Member]
CURRENT ASSETS :
Cash and cash equivalents	2,490	2,727	568	247
Short-term investments	217	165
Receivables, net	0	0
Fiduciary assets	0	0
Intercompany receivables	21,073	2,814
Other current assets	221	226
Total Current Assets	24,001	5,932
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Intercompany receivables	557	600
Other non-current assets	698	697
Investment in subsidiary	16,060	15,200
TOTAL ASSETS	41,316	22,429
CURRENT LIABILITIES
Fiduciary liabilities	0	0
Short-term debt and current portion of long-term debt	1,254	767
Accounts payable and accrued liabilities	25	58
Intercompany payables	27,826	8,960
Other current liabilities	55	49
Total Current Liabilities	29,160	9,834
Long-term debt	1,418	1,917
Pension, other post-retirement and post-employment liabilities	1,293	1,396
Intercompany payables	8,796	7,277
Other non-current liabilities	99	125
TOTAL LIABILITIES	40,766	20,549
TOTAL AON SHAREHOLDERS' EQUITY	550	1,880
Noncontrolling interests	0	0
TOTAL EQUITY	550	1,880
TOTAL LIABILITIES AND EQUITY	41,316	22,429
Other Non-Guarantor Subsidiaries [Member]
CURRENT ASSETS :
Cash and cash equivalents	1,087	1,361	1,444	1,246
Short-term investments	168	229
Receivables, net	2,453	2,815
Fiduciary assets	9,956	11,638
Intercompany receivables	2,402	9,156
Other current assets	532	407
Total Current Assets	16,598	25,606
Goodwill	8,571	8,860
Intangible assets, net	2,257	2,520
Fixed assets, net	785	765
Intercompany receivables	1,629	111
Other non-current assets	1,572	1,718
Investment in subsidiary	550	1,880
TOTAL ASSETS	31,962	41,460
CURRENT LIABILITIES
Fiduciary liabilities	9,956	11,638
Short-term debt and current portion of long-term debt	17	16
Accounts payable and accrued liabilities	1,361	1,706
Intercompany payables	(4,142)	3,343
Other current liabilities	657	772
Total Current Liabilities	7,849	17,475
Long-term debt	287	338
Pension, other post-retirement and post-employment liabilities	540	745
Intercompany payables	770	833
Other non-current liabilities	1,770	1,847
TOTAL LIABILITIES	11,216	21,238
TOTAL AON SHAREHOLDERS' EQUITY	20,687	20,162
Noncontrolling interests	59	60
TOTAL EQUITY	20,746	20,222
TOTAL LIABILITIES AND EQUITY	31,962	41,460
Consolidating Adjustments [Member]
CURRENT ASSETS :
Cash and cash equivalents	(3,179)	(3,714)	$ (1,630)	$ (1,016)
Short-term investments	0	0
Receivables, net	0	0
Fiduciary assets	0	0
Intercompany receivables	(23,835)	(12,425)
Other current assets	(27)	(33)
Total Current Assets	(27,041)	(16,172)
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Intercompany receivables	(9,566)	(8,110)
Other non-current assets	(784)	(790)
Investment in subsidiary	(21,237)	(22,042)
TOTAL ASSETS	(58,628)	(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	(3,179)	(3,714)
Intercompany payables	(23,961)	(12,425)
Other current liabilities	99	(33)
Total Current Liabilities	(27,041)	(16,172)
Long-term debt	0	0
Pension, other post-retirement and post-employment liabilities	0	0
Intercompany payables	(9,566)	(8,110)
Other non-current liabilities	(784)	(790)
TOTAL LIABILITIES	(37,391)	(25,072)
TOTAL AON SHAREHOLDERS' EQUITY	(21,237)	(22,042)
Noncontrolling interests	0	0
TOTAL EQUITY	(21,237)	(22,042)
TOTAL LIABILITIES AND EQUITY	$ (58,628)	$ (47,114)