Guarantee of Registered Securities - Condensed Consolidating Statement of Financial Position (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Total Current Assets
Cash and cash equivalents	$ 431	$ 384	$ 374	$ 477
Short-term investments	290	356
Receivables, net	2,589	2,564
Fiduciary assets	9,485	9,932
Intercompany receivables	0	0
Other current assets	351	329
Total Current Assets	13,146	13,565
Goodwill	8,747	8,448	8,860
Intangible assets, net	2,223	2,180
Fixed assets, net	765	765
Deferred tax assets	322	300
Intercompany receivables	0	0
Prepaid pension	858	1,033
Other non-current assets	554	592
Investment in subsidiary	0	0
TOTAL ASSETS	26,615	26,883
CURRENT LIABILITIES
Accounts payable and accrued liabilities	1,801	1,772
Short-term debt and current portion of long-term debt	336	562
Fiduciary liabilities	9,485	9,932
Intercompany payables	0	0
Other current liabilities	873	819
Total Current Liabilities	12,495	13,085
Long-term debt	5,869	5,138
Deferred tax liabilities	101	37
Pension, other post-retirement and other post-employment liabilities	1,774	1,795
Intercompany payables	0	0
Other non-current liabilities	844	769
TOTAL LIABILITIES	21,083	20,824
TOTAL AON SHAREHOLDERS’ EQUITY	5,475	6,002
Noncontrolling interests	57	57
TOTAL EQUITY	5,532	6,059	6,527	8,091
TOTAL LIABILITIES AND EQUITY	26,615	26,883
Aon plc
Total 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	105	432
Other current assets	0	0
Total Current Assets	105	433
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Deferred tax assets	134	154
Intercompany receivables	366	375
Prepaid pension	0	0
Other non-current assets	2	0
Investment in subsidiary	10,107	11,700
TOTAL ASSETS	10,714	12,662
CURRENT LIABILITIES
Accounts payable and accrued liabilities	585	2,988
Short-term debt and current portion of long-term debt	279	0
Fiduciary liabilities	0	0
Intercompany payables	142	167
Other current liabilities	0	47
Total Current Liabilities	1,006	3,202
Long-term debt	4,177	3,451
Deferred tax liabilities	0	0
Pension, other post-retirement and other post-employment liabilities	0	0
Intercompany payables	0	0
Other non-current liabilities	8	7
TOTAL LIABILITIES	5,191	6,660
TOTAL AON SHAREHOLDERS’ EQUITY	5,523	6,002
Noncontrolling interests	0	0
TOTAL EQUITY	5,523	6,002
TOTAL LIABILITIES AND EQUITY	10,714	12,662
Aon Corporation
Total Current Assets
Cash and cash equivalents	1,633	2,083	2,727	247
Short-term investments	140	209
Receivables, net	3	0
Fiduciary assets	0	0
Intercompany receivables	1,880	1,950
Other current assets	25	19
Total Current Assets	3,681	4,261
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Deferred tax assets	726	756
Intercompany receivables	261	526
Prepaid pension	5	6
Other non-current assets	119	119
Investment in subsidiary	17,137	16,042
TOTAL ASSETS	21,929	21,710
CURRENT LIABILITIES
Accounts payable and accrued liabilities	44	45
Short-term debt and current portion of long-term debt	50	550
Fiduciary liabilities	0	0
Intercompany payables	10,399	9,518
Other current liabilities	63	56
Total Current Liabilities	10,556	10,169
Long-term debt	1,413	1,412
Deferred tax liabilities	0	0
Pension, other post-retirement and other post-employment liabilities	1,356	1,313
Intercompany payables	8,877	8,799
Other non-current liabilities	77	140
TOTAL LIABILITIES	22,279	21,833
TOTAL AON SHAREHOLDERS’ EQUITY	(350)	(123)
Noncontrolling interests	0	0
TOTAL EQUITY	(350)	(123)
TOTAL LIABILITIES AND EQUITY	21,929	21,710
Other Non-Guarantor Subsidiaries
Total Current Assets
Cash and cash equivalents	660	1,242	1,361	1,246
Short-term investments	150	147
Receivables, net	2,586	2,563
Fiduciary assets	9,485	9,932
Intercompany receivables	9,825	7,957
Other current assets	326	310
Total Current Assets	23,032	22,151
Goodwill	8,747	8,448
Intangible assets, net	2,223	2,180
Fixed assets, net	765	765
Deferred tax assets	168	207
Intercompany receivables	8,711	8,633
Prepaid pension	853	1,027
Other non-current assets	433	557
Investment in subsidiary	(350)	(123)
TOTAL ASSETS	44,582	43,845
CURRENT LIABILITIES
Accounts payable and accrued liabilities	3,034	1,680
Short-term debt and current portion of long-term debt	7	12
Fiduciary liabilities	9,485	9,932
Intercompany payables	1,269	654
Other current liabilities	810	716
Total Current Liabilities	14,605	12,994
Long-term debt	279	275
Deferred tax liabilities	759	855
Pension, other post-retirement and other post-employment liabilities	418	482
Intercompany payables	461	735
Other non-current liabilities	759	705
TOTAL LIABILITIES	17,281	16,046
TOTAL AON SHAREHOLDERS’ EQUITY	27,244	27,742
Noncontrolling interests	57	57
TOTAL EQUITY	27,301	27,799
TOTAL LIABILITIES AND EQUITY	44,582	43,845
Consolidating Adjustments
Total Current Assets
Cash and cash equivalents	(1,862)	(2,941)	$ (3,714)	$ (1,016)
Short-term investments	0	0
Receivables, net	0	0
Fiduciary assets	0	0
Intercompany receivables	(11,810)	(10,339)
Other current assets	0	0
Total Current Assets	(13,672)	(13,280)
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Deferred tax assets	(706)	(817)
Intercompany receivables	(9,338)	(9,534)
Prepaid pension	0	0
Other non-current assets	0	(84)
Investment in subsidiary	(26,894)	(27,619)
TOTAL ASSETS	(50,610)	(51,334)
CURRENT LIABILITIES
Accounts payable and accrued liabilities	(1,862)	(2,941)
Short-term debt and current portion of long-term debt	0	0
Fiduciary liabilities	0	0
Intercompany payables	(11,810)	(10,339)
Other current liabilities	0	0
Total Current Liabilities	(13,672)	(13,280)
Long-term debt	0	0
Deferred tax liabilities	(658)	(818)
Pension, other post-retirement and other post-employment liabilities	0	0
Intercompany payables	(9,338)	(9,534)
Other non-current liabilities	0	(83)
TOTAL LIABILITIES	(23,668)	(23,715)
TOTAL AON SHAREHOLDERS’ EQUITY	(26,942)	(27,619)
Noncontrolling interests	0	0
TOTAL EQUITY	(26,942)	(27,619)
TOTAL LIABILITIES AND EQUITY	$ (50,610)	$ (51,334)