Guarantee of Registered Securities - Condensed Consolidating Statement of Financial Position (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2014
CURRENT ASSETS :
Cash and cash equivalents	$ 465	$ 384	$ 378	$ 374
Short-term investments	587	356
Receivables, net	2,591	2,734
Fiduciary assets	9,776	9,932
Intercompany receivables	0	0
Other current assets	622	562
Total Current Assets	14,041	13,968
Goodwill	8,411	8,448
Intangible assets, net	2,108	2,180
Fixed assets, net	766	765
Intercompany receivables	0	0
Non-current deferred tax assets	171	141
Prepaid pension	737	1,033
Other non-current assets	579	592
Investment in subsidiary	0	0
TOTAL ASSETS	26,813	27,127
CURRENT LIABILITIES
Accounts payable and accrued liabilities	9,776	9,932
Short-term debt and current portion of long-term debt	695	562
Accounts payable and accrued liabilities	1,420	1,772
Intercompany payables	0	0
Other current liabilities	902	820
Total Current Liabilities	12,793	13,086
Long-term debt	5,902	5,138
Non-current deferred tax liabilities	177	176
Pension, other post-retirement and post-employment liabilities	1,756	1,795
Intercompany payables	0	0
Other non-current liabilities	838	769
TOTAL LIABILITIES	21,466	20,964
TOTAL AON SHAREHOLDERS' EQUITY	5,281	6,106
Noncontrolling interests	66	57
TOTAL EQUITY	5,347	6,163
TOTAL LIABILITIES AND EQUITY	26,813	27,127
Aon plc [Member]
CURRENT ASSETS :
Cash and cash equivalents	0	0	0	0
Short-term investments	0	0
Receivables, net	2	1
Fiduciary assets	0	0
Intercompany receivables	43	432
Other current assets	0	0
Total Current Assets	45	433
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Intercompany receivables	379	375
Non-current deferred tax assets	154	154
Prepaid pension	0	0
Other non-current assets	0	0
Investment in subsidiary	11,949	11,804
TOTAL ASSETS	12,527	12,766
CURRENT LIABILITIES
Accounts payable and accrued liabilities	0	0
Short-term debt and current portion of long-term debt	184	0
Accounts payable and accrued liabilities	2,624	2,988
Intercompany payables	141	167
Other current liabilities	85	47
Total Current Liabilities	3,034	3,202
Long-term debt	4,206	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	7,246	6,660
TOTAL AON SHAREHOLDERS' EQUITY	5,281	6,106
Noncontrolling interests	0	0
TOTAL EQUITY	5,281	6,106
TOTAL LIABILITIES AND EQUITY	12,527	12,766
Aon Corporation [Member]
CURRENT ASSETS :
Cash and cash equivalents	1,537	2,083	2,230	2,727
Short-term investments	453	209
Receivables, net	0	0
Fiduciary assets	0	0
Intercompany receivables	3,786	1,950
Other current assets	206	217
Total Current Assets	5,982	4,459
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Intercompany receivables	537	526
Non-current deferred tax assets	556	558
Prepaid pension	6	6
Other non-current assets	114	119
Investment in subsidiary	16,643	16,534
TOTAL ASSETS	23,838	22,202
CURRENT LIABILITIES
Accounts payable and accrued liabilities	0	0
Short-term debt and current portion of long-term debt	500	550
Accounts payable and accrued liabilities	22	45
Intercompany payables	11,365	9,518
Other current liabilities	60	56
Total Current Liabilities	11,947	10,169
Long-term debt	1,413	1,412
Non-current deferred tax liabilities	0	0
Pension, other post-retirement and post-employment liabilities	1,295	1,313
Intercompany payables	8,881	8,799
Other non-current liabilities	63	140
TOTAL LIABILITIES	23,599	21,833
TOTAL AON SHAREHOLDERS' EQUITY	239	369
Noncontrolling interests	0	0
TOTAL EQUITY	239	369
TOTAL LIABILITIES AND EQUITY	23,838	22,202
Other Non-Guarantor Subsidiaries [Member]
CURRENT ASSETS :
Cash and cash equivalents	1,457	1,242	650	1,361
Short-term investments	134	147
Receivables, net	2,589	2,733
Fiduciary assets	9,776	9,932
Intercompany receivables	8,759	7,957
Other current assets	418	347
Total Current Assets	23,133	22,358
Goodwill	8,411	8,448
Intangible assets, net	2,108	2,180
Fixed assets, net	766	765
Intercompany receivables	8,716	8,633
Non-current deferred tax assets	137	107
Prepaid pension	731	1,027
Other non-current assets	465	557
Investment in subsidiary	239	369
TOTAL ASSETS	44,706	44,444
CURRENT LIABILITIES
Accounts payable and accrued liabilities	9,776	9,932
Short-term debt and current portion of long-term debt	11	12
Accounts payable and accrued liabilities	1,303	1,680
Intercompany payables	1,082	654
Other current liabilities	759	720
Total Current Liabilities	12,931	12,998
Long-term debt	283	275
Non-current deferred tax liabilities	853	854
Pension, other post-retirement and post-employment liabilities	461	482
Intercompany payables	751	735
Other non-current liabilities	769	705
TOTAL LIABILITIES	16,048	16,049
TOTAL AON SHAREHOLDERS' EQUITY	28,592	28,338
Noncontrolling interests	66	57
TOTAL EQUITY	28,658	28,395
TOTAL LIABILITIES AND EQUITY	44,706	44,444
Consolidating Adjustments [Member]
CURRENT ASSETS :
Cash and cash equivalents	(2,529)	(2,941)	$ (2,502)	$ (3,714)
Short-term investments	0	0
Receivables, net	0	0
Fiduciary assets	0	0
Intercompany receivables	(12,588)	(10,339)
Other current assets	(2)	(2)
Total Current Assets	(15,119)	(13,282)
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Intercompany receivables	(9,632)	(9,534)
Non-current deferred tax assets	(676)	(678)
Prepaid pension	0	0
Other non-current assets	0	(84)
Investment in subsidiary	(28,831)	(28,707)
TOTAL ASSETS	(54,258)	(52,285)
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	(2,529)	(2,941)
Intercompany payables	(12,588)	(10,339)
Other current liabilities	(2)	(3)
Total Current Liabilities	(15,119)	(13,283)
Long-term debt	0	0
Non-current deferred tax liabilities	(676)	(678)
Pension, other post-retirement and post-employment liabilities	0	0
Intercompany payables	(9,632)	(9,534)
Other non-current liabilities	0	(83)
TOTAL LIABILITIES	(25,427)	(23,578)
TOTAL AON SHAREHOLDERS' EQUITY	(28,831)	(28,707)
Noncontrolling interests	0	0
TOTAL EQUITY	(28,831)	(28,707)
TOTAL LIABILITIES AND EQUITY	$ (54,258)	$ (52,285)