Guarantee of Registered Securities - Condensed Consolidating Statement of Financial Position (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015	Dec. 31, 2014
CURRENT ASSETS :
Cash and cash equivalents	$ 438	$ 384	$ 367	$ 374
Short-term investments	251	356
Receivables, net	2,516	2,734
Fiduciary assets	10,201	9,932
Intercompany receivables	0	0
Other current assets	395	329
Total Current Assets	13,801	13,735
Goodwill	8,473	8,448
Intangible assets, net	2,107	2,180
Fixed assets, net	751	765
Intercompany receivables	0	0
Non-current deferred tax assets	265	234
Prepaid pension	720	1,033
Other non-current assets	576	592
Investment in subsidiary	0	0
TOTAL ASSETS	26,693	26,987
CURRENT LIABILITIES
Accounts payable and accrued liabilities	10,201	9,932
Short-term debt and current portion of long-term debt	251	562
Accounts payable and accrued liabilities	1,373	1,772
Intercompany payables	0	0
Other current liabilities	872	819
Total Current Liabilities	12,697	13,085
Long-term debt	5,907	5,138
Non-current deferred tax liabilities	39	37
Pension, other post-retirement and post-employment liabilities	1,722	1,795
Intercompany payables	0	0
Other non-current liabilities	767	769
TOTAL LIABILITIES	21,132	20,824
TOTAL AON SHAREHOLDERS' EQUITY	5,499	6,106
Noncontrolling interests	62	57
TOTAL EQUITY	5,561	6,163
TOTAL LIABILITIES AND EQUITY	26,693	26,987
Aon plc [Member]
CURRENT ASSETS :
Cash and cash equivalents	0	0	0	0
Short-term investments	0	0
Receivables, net	0	1
Fiduciary assets	0	0
Intercompany receivables	55	432
Other current assets	0	0
Total Current Assets	55	433
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Intercompany receivables	378	375
Non-current deferred tax assets	154	154
Prepaid pension	0	0
Other non-current assets	0	0
Investment in subsidiary	12,315	11,804
TOTAL ASSETS	12,902	12,766
CURRENT LIABILITIES
Accounts payable and accrued liabilities	0	0
Short-term debt and current portion of long-term debt	241	0
Accounts payable and accrued liabilities	2,803	2,988
Intercompany payables	147	167
Other current liabilities	1	47
Total Current Liabilities	3,192	3,202
Long-term debt	4,205	3,451
Non-current deferred tax liabilities	0	0
Pension, other post-retirement and post-employment liabilities	0	0
Intercompany payables	0	0
Other non-current liabilities	6	7
TOTAL LIABILITIES	7,403	6,660
TOTAL AON SHAREHOLDERS' EQUITY	5,499	6,106
Noncontrolling interests	0	0
TOTAL EQUITY	5,499	6,106
TOTAL LIABILITIES AND EQUITY	12,902	12,766
Aon Corporation [Member]
CURRENT ASSETS :
Cash and cash equivalents	1,349	2,083	2,221	2,727
Short-term investments	118	209
Receivables, net	1	0
Fiduciary assets	0	0
Intercompany receivables	4,847	1,950
Other current assets	24	19
Total Current Assets	6,339	4,261
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Intercompany receivables	262	526
Non-current deferred tax assets	746	756
Prepaid pension	6	6
Other non-current assets	111	119
Investment in subsidiary	16,914	16,534
TOTAL ASSETS	24,378	22,202
CURRENT LIABILITIES
Accounts payable and accrued liabilities	0	0
Short-term debt and current portion of long-term debt	0	550
Accounts payable and accrued liabilities	45	45
Intercompany payables	12,357	9,518
Other current liabilities	60	56
Total Current Liabilities	12,462	10,169
Long-term debt	1,413	1,412
Non-current deferred tax liabilities	0	0
Pension, other post-retirement and post-employment liabilities	1,284	1,313
Intercompany payables	8,878	8,799
Other non-current liabilities	64	140
TOTAL LIABILITIES	24,101	21,833
TOTAL AON SHAREHOLDERS' EQUITY	277	369
Noncontrolling interests	0	0
TOTAL EQUITY	277	369
TOTAL LIABILITIES AND EQUITY	24,378	22,202
Other Non-Guarantor Subsidiaries [Member]
CURRENT ASSETS :
Cash and cash equivalents	1,850	1,242	707	1,361
Short-term investments	133	147
Receivables, net	2,515	2,733
Fiduciary assets	10,201	9,932
Intercompany receivables	9,436	7,957
Other current assets	371	310
Total Current Assets	24,506	22,321
Goodwill	8,473	8,448
Intangible assets, net	2,107	2,180
Fixed assets, net	751	765
Intercompany receivables	8,711	8,633
Non-current deferred tax assets	176	141
Prepaid pension	714	1,027
Other non-current assets	465	557
Investment in subsidiary	277	369
TOTAL ASSETS	46,180	44,441
CURRENT LIABILITIES
Accounts payable and accrued liabilities	10,201	9,932
Short-term debt and current portion of long-term debt	10	12
Accounts payable and accrued liabilities	1,286	1,680
Intercompany payables	1,834	654
Other current liabilities	811	716
Total Current Liabilities	14,142	12,994
Long-term debt	289	275
Non-current deferred tax liabilities	850	855
Pension, other post-retirement and post-employment liabilities	438	482
Intercompany payables	473	735
Other non-current liabilities	697	705
TOTAL LIABILITIES	16,889	16,046
TOTAL AON SHAREHOLDERS' EQUITY	29,229	28,338
Noncontrolling interests	62	57
TOTAL EQUITY	29,291	28,395
TOTAL LIABILITIES AND EQUITY	46,180	44,441
Consolidating Adjustments [Member]
CURRENT ASSETS :
Cash and cash equivalents	(2,761)	(2,941)	$ (2,561)	$ (3,714)
Short-term investments	0	0
Receivables, net	0	0
Fiduciary assets	0	0
Intercompany receivables	(14,338)	(10,339)
Other current assets	0	0
Total Current Assets	(17,099)	(13,280)
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Intercompany receivables	(9,351)	(9,534)
Non-current deferred tax assets	(811)	(817)
Prepaid pension	0	0
Other non-current assets	0	(84)
Investment in subsidiary	(29,506)	(28,707)
TOTAL ASSETS	(56,767)	(52,422)
CURRENT LIABILITIES
Accounts payable and accrued liabilities	0	0
Short-term debt and current portion of long-term debt	0	0
Accounts payable and accrued liabilities	(2,761)	(2,941)
Intercompany payables	(14,338)	(10,339)
Other current liabilities	0	0
Total Current Liabilities	(17,099)	(13,280)
Long-term debt	0	0
Non-current deferred tax liabilities	(811)	(818)
Pension, other post-retirement and post-employment liabilities	0	0
Intercompany payables	(9,351)	(9,534)
Other non-current liabilities	0	(83)
TOTAL LIABILITIES	(27,261)	(23,715)
TOTAL AON SHAREHOLDERS' EQUITY	(29,506)	(28,707)
Noncontrolling interests	0	0
TOTAL EQUITY	(29,506)	(28,707)
TOTAL LIABILITIES AND EQUITY	$ (56,767)	$ (52,422)