Guarantee of Registered Securities - Condensed Consolidating Statement of Financial Position (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2014
CURRENT ASSETS :
Cash and cash equivalents	$ 483	$ 384	$ 398	$ 374
Short-term investments	463	356
Receivables, net	2,391	2,734
Fiduciary assets	8,710	9,932
Intercompany receivables	0	0
Other current assets	424	329
Total Current Assets	12,471	13,735
Goodwill	8,452	8,448
Intangible assets, net	2,038	2,180
Fixed assets, net	738	765
Intercompany receivables	0	0
Non-current deferred tax assets	278	234
Prepaid pension	711	1,033
Other non-current assets	564	592
Investment in subsidiary	0	0
TOTAL ASSETS	25,252	26,987
CURRENT LIABILITIES
Accounts payable and accrued liabilities	8,710	9,932
Short-term debt and current portion of long-term debt	250	562
Accounts payable and accrued liabilities	1,477	1,772
Intercompany payables	0	0
Other current liabilities	929	819
Total Current Liabilities	11,366	13,085
Long-term debt	5,910	5,138
Non-current deferred tax liabilities	36	37
Pension, other post-retirement and post-employment liabilities	1,687	1,795
Intercompany payables	0	0
Other non-current liabilities	759	769
TOTAL LIABILITIES	19,758	20,824
TOTAL AON SHAREHOLDERS' EQUITY	5,434	6,106
Noncontrolling interests	60	57
TOTAL EQUITY	5,494	6,163
TOTAL LIABILITIES AND EQUITY	25,252	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	63	432
Other current assets	0	0
Total Current Assets	63	433
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Intercompany receivables	381	375
Non-current deferred tax assets	154	154
Prepaid pension	0	0
Other non-current assets	0	0
Investment in subsidiary	12,313	11,804
TOTAL ASSETS	12,911	12,766
CURRENT LIABILITIES
Accounts payable and accrued liabilities	0	0
Short-term debt and current portion of long-term debt	244	0
Accounts payable and accrued liabilities	2,865	2,988
Intercompany payables	134	167
Other current liabilities	14	47
Total Current Liabilities	3,257	3,202
Long-term debt	4,213	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	7	7
TOTAL LIABILITIES	7,477	6,660
TOTAL AON SHAREHOLDERS' EQUITY	5,434	6,106
Noncontrolling interests	0	0
TOTAL EQUITY	5,434	6,106
TOTAL LIABILITIES AND EQUITY	12,911	12,766
Aon Corporation [Member]
CURRENT ASSETS :
Cash and cash equivalents	1,119	2,083	2,490	2,727
Short-term investments	308	209
Receivables, net	0	0
Fiduciary assets	0	0
Intercompany receivables	5,740	1,950
Other current assets	29	19
Total Current Assets	7,196	4,261
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Intercompany receivables	262	526
Non-current deferred tax assets	737	756
Prepaid pension	6	6
Other non-current assets	126	119
Investment in subsidiary	17,115	16,534
TOTAL ASSETS	25,442	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	18	45
Intercompany payables	13,716	9,518
Other current liabilities	61	56
Total Current Liabilities	13,795	10,169
Long-term debt	1,414	1,412
Non-current deferred tax liabilities	0	0
Pension, other post-retirement and post-employment liabilities	1,275	1,313
Intercompany payables	8,879	8,799
Other non-current liabilities	70	140
TOTAL LIABILITIES	25,433	21,833
TOTAL AON SHAREHOLDERS' EQUITY	9	369
Noncontrolling interests	0	0
TOTAL EQUITY	9	369
TOTAL LIABILITIES AND EQUITY	25,442	22,202
Other Non-Guarantor Subsidiaries [Member]
CURRENT ASSETS :
Cash and cash equivalents	2,157	1,242	1,087	1,361
Short-term investments	155	147
Receivables, net	2,391	2,733
Fiduciary assets	8,710	9,932
Intercompany receivables	10,320	7,957
Other current assets	395	310
Total Current Assets	24,128	22,321
Goodwill	8,452	8,448
Intangible assets, net	2,038	2,180
Fixed assets, net	738	765
Intercompany receivables	8,712	8,633
Non-current deferred tax assets	164	141
Prepaid pension	705	1,027
Other non-current assets	438	557
Investment in subsidiary	9	369
TOTAL ASSETS	45,384	44,441
CURRENT LIABILITIES
Accounts payable and accrued liabilities	8,710	9,932
Short-term debt and current portion of long-term debt	6	12
Accounts payable and accrued liabilities	1,387	1,680
Intercompany payables	2,273	654
Other current liabilities	854	716
Total Current Liabilities	13,230	12,994
Long-term debt	283	275
Non-current deferred tax liabilities	813	855
Pension, other post-retirement and post-employment liabilities	412	482
Intercompany payables	476	735
Other non-current liabilities	682	705
TOTAL LIABILITIES	15,896	16,046
TOTAL AON SHAREHOLDERS' EQUITY	29,428	28,338
Noncontrolling interests	60	57
TOTAL EQUITY	29,488	28,395
TOTAL LIABILITIES AND EQUITY	45,384	44,441
Consolidating Adjustments [Member]
CURRENT ASSETS :
Cash and cash equivalents	(2,793)	(2,941)	$ (3,179)	$ (3,714)
Short-term investments	0	0
Receivables, net	0	0
Fiduciary assets	0	0
Intercompany receivables	(16,123)	(10,339)
Other current assets	0	0
Total Current Assets	(18,916)	(13,280)
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Intercompany receivables	(9,355)	(9,534)
Non-current deferred tax assets	(777)	(817)
Prepaid pension	0	0
Other non-current assets	0	(84)
Investment in subsidiary	(29,437)	(28,707)
TOTAL ASSETS	(58,485)	(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	(2,793)	(2,941)
Intercompany payables	(16,123)	(10,339)
Other current liabilities		0
Total Current Liabilities	(18,916)	(13,280)
Long-term debt	0	0
Non-current deferred tax liabilities	(777)	(818)
Pension, other post-retirement and post-employment liabilities	0	0
Intercompany payables	(9,355)	(9,534)
Other non-current liabilities	0	(83)
TOTAL LIABILITIES	(29,048)	(23,715)
TOTAL AON SHAREHOLDERS' EQUITY	(29,437)	(28,707)
Noncontrolling interests	0	0
TOTAL EQUITY	(29,437)	(28,707)
TOTAL LIABILITIES AND EQUITY	$ (58,485)	$ (52,422)