Guarantee of Registered Securities - Condensed Consolidating Statement of Financial Position (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014	Dec. 31, 2013
CURRENT ASSETS :
Cash and cash equivalents	$ 367	$ 374	$ 418	$ 477
Short-term investments	484	394
Receivables, net	2,684	2,815
Fiduciary assets	11,389	11,638
Intercompany receivables	0	0
Other current assets	731	602
Total Current Assets	15,655	15,823
Goodwill	8,661	8,860
Intangible assets, net	2,350	2,520
Fixed assets, net	782	765
Intercompany receivables	0	0
Other non-current assets	1,755	1,804
Investment in subsidiary	0	0
TOTAL ASSETS	29,203	29,772
CURRENT LIABILITIES
Fiduciary liabilities	11,389	11,638
Short-term debt and current portion of long-term debt	1,237	783
Accounts payable and accrued liabilities	1,368	1,805
Intercompany payables	0	0
Other current liabilities	845	788
Total Current Liabilities	14,839	15,014
Long-term debt	4,824	4,799
Pension, other post-retirement and post-employment liabilities	1,944	2,141
Intercompany payables	0	0
Other non-current liabilities	1,140	1,187
TOTAL LIABILITIES	22,747	23,141
TOTAL AON SHAREHOLDERS' EQUITY	6,398	6,571
Noncontrolling interests	58	60
TOTAL EQUITY	6,456	6,631
TOTAL LIABILITIES AND EQUITY	29,203	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	258	455
Other current assets	2	2
Total Current Assets	260	457
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Intercompany receivables	7,379	7,399
Other non-current assets	189	179
Investment in subsidiary	4,488	4,962
TOTAL ASSETS	12,316	12,997
CURRENT LIABILITIES
Fiduciary liabilities	0	0
Short-term debt and current portion of long-term debt	0	0
Accounts payable and accrued liabilities	2,597	3,755
Intercompany payables	223	122
Other current liabilities	0	0
Total Current Liabilities	2,820	3,877
Long-term debt	3,092	2,544
Pension, other post-retirement and post-employment liabilities	0	0
Intercompany payables	0	0
Other non-current liabilities	6	5
TOTAL LIABILITIES	5,918	6,426
TOTAL AON SHAREHOLDERS' EQUITY	6,398	6,571
Noncontrolling interests	0	0
TOTAL EQUITY	6,398	6,571
TOTAL LIABILITIES AND EQUITY	12,316	12,997
Aon Corporation [Member]
CURRENT ASSETS :
Cash and cash equivalents	2,221	2,727	181	247
Short-term investments	125	165
Receivables, net	0	0
Fiduciary assets	0	0
Intercompany receivables	3,847	2,814
Other current assets	223	226
Total Current Assets	6,416	5,932
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Intercompany receivables	581	600
Other non-current assets	693	697
Investment in subsidiary	15,799	15,200
TOTAL ASSETS	23,489	22,429
CURRENT LIABILITIES
Fiduciary liabilities	0	0
Short-term debt and current portion of long-term debt	1,225	767
Accounts payable and accrued liabilities	49	58
Intercompany payables	9,975	8,960
Other current liabilities	55	49
Total Current Liabilities	11,304	9,834
Long-term debt	1,418	1,917
Pension, other post-retirement and post-employment liabilities	1,336	1,396
Intercompany payables	8,787	7,277
Other non-current liabilities	102	125
TOTAL LIABILITIES	22,947	20,549
TOTAL AON SHAREHOLDERS' EQUITY	542	1,880
Noncontrolling interests	0	0
TOTAL EQUITY	542	1,880
TOTAL LIABILITIES AND EQUITY	23,489	22,429
Other Non-Guarantor Subsidiaries [Member]
CURRENT ASSETS :
Cash and cash equivalents	707	1,361	1,416	1,246
Short-term investments	359	229
Receivables, net	2,684	2,815
Fiduciary assets	11,389	11,638
Intercompany receivables	10,286	9,156
Other current assets	538	407
Total Current Assets	25,963	25,606
Goodwill	8,661	8,860
Intangible assets, net	2,350	2,520
Fixed assets, net	782	765
Intercompany receivables	1,621	111
Other non-current assets	1,656	1,718
Investment in subsidiary	542	1,880
TOTAL ASSETS	41,575	41,460
CURRENT LIABILITIES
Fiduciary liabilities	11,389	11,638
Short-term debt and current portion of long-term debt	12	16
Accounts payable and accrued liabilities	1,283	1,706
Intercompany payables	4,194	3,343
Other current liabilities	822	772
Total Current Liabilities	17,700	17,475
Long-term debt	314	338
Pension, other post-retirement and post-employment liabilities	608	745
Intercompany payables	794	833
Other non-current liabilities	1,814	1,847
TOTAL LIABILITIES	21,230	21,238
TOTAL AON SHAREHOLDERS' EQUITY	20,287	20,162
Noncontrolling interests	58	60
TOTAL EQUITY	20,345	20,222
TOTAL LIABILITIES AND EQUITY	41,575	41,460
Consolidating Adjustments [Member]
CURRENT ASSETS :
Cash and cash equivalents	(2,561)	(3,714)	$ (1,179)	$ (1,016)
Short-term investments	0	0
Receivables, net	0	0
Fiduciary assets	0	0
Intercompany receivables	(14,391)	(12,425)
Other current assets	(32)	(33)
Total Current Assets	(16,984)	(16,172)
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Intercompany receivables	(9,581)	(8,110)
Other non-current assets	(783)	(790)
Investment in subsidiary	(20,829)	(22,042)
TOTAL ASSETS	(48,177)	(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	(2,561)	(3,714)
Intercompany payables	(14,392)	(12,425)
Other current liabilities	(32)	(33)
Total Current Liabilities	(16,985)	(16,172)
Long-term debt	0	0
Pension, other post-retirement and post-employment liabilities	0	0
Intercompany payables	(9,581)	(8,110)
Other non-current liabilities	(782)	(790)
TOTAL LIABILITIES	(27,348)	(25,072)
TOTAL AON SHAREHOLDERS' EQUITY	(20,829)	(22,042)
Noncontrolling interests	0	0
TOTAL EQUITY	(20,829)	(22,042)
TOTAL LIABILITIES AND EQUITY	$ (48,177)	$ (47,114)