Guarantee of Registered Securities - Condensed Consolidating Statement of Financial Position (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
CURRENT ASSETS :
Cash and cash equivalents	$ 433	$ 426
Short-term investments	200	290
Receivables, net	2,103	2,106
Fiduciary assets	9,162	8,959
Intercompany receivables	0	0
Current assets of discontinued operations	3,186	1,118
Other current assets	309	247
Total Current Assets	15,393	13,146
Goodwill	7,544	7,410
Intangible assets, net	1,886	1,890
Fixed assets, net	536	550
Deferred tax assets	351	325
Intercompany receivables	0	0
Prepaid pension	893	858
Other non-current assets	379	360
Investment in subsidiary	0	0
Non-current assets of discontinued operations	0	2,076
TOTAL ASSETS	26,982	26,615
CURRENT LIABILITIES
Accounts payable and accrued liabilities	1,332	1,604
Short-term debt and current portion of long-term debt	667	336
Fiduciary liabilities	9,162	8,959
Intercompany payables	0	0
Current liabilities of discontinued operations	1,036	940
Other current liabilities	773	656
Total Current Liabilities	12,970	12,495
Long-term debt	5,610	5,869
Non-current deferred tax liabilities	112	101
Pension, other postretirement and postemployment liabilities	1,731	1,760
Intercompany payables	0	0
Other non-current liabilities	733	719
Non-current liabilities of discontinued operations	0	139
TOTAL LIABILITIES	21,156	21,083
TOTAL AON SHAREHOLDERS’ EQUITY	5,754	5,475
Noncontrolling interests	72	57
TOTAL EQUITY	5,826	5,532
TOTAL LIABILITIES AND EQUITY	26,982	26,615
Aon plc
CURRENT ASSETS :
Cash and cash equivalents	0	0
Short-term investments	0	0
Receivables, net	0	0
Fiduciary assets	0	0
Intercompany receivables	73	105
Current assets of discontinued operations	0	0
Other current assets	6	0
Total Current Assets	79	105
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Deferred tax assets	134	134
Intercompany receivables	372	366
Prepaid pension	0	0
Other non-current assets	2	2
Investment in subsidiary	10,707	10,107
Non-current assets of discontinued operations		0
TOTAL ASSETS	11,294	10,714
CURRENT LIABILITIES
Accounts payable and accrued liabilities	837	585
Short-term debt and current portion of long-term debt	324	279
Fiduciary liabilities	0	0
Intercompany payables	167	142
Current liabilities of discontinued operations	0	0
Other current liabilities	0	0
Total Current Liabilities	1,328	1,006
Long-term debt	4,196	4,177
Non-current deferred tax liabilities	0	0
Pension, other postretirement and postemployment liabilities	0	0
Intercompany payables	0	0
Other non-current liabilities	16	8
Non-current liabilities of discontinued operations		0
TOTAL LIABILITIES	5,540	5,191
TOTAL AON SHAREHOLDERS’ EQUITY	5,754	5,523
Noncontrolling interests	0	0
TOTAL EQUITY	5,754	5,523
TOTAL LIABILITIES AND EQUITY	11,294	10,714
Aon Corporation
CURRENT ASSETS :
Cash and cash equivalents	1,425	1,633
Short-term investments	46	140
Receivables, net	0	3
Fiduciary assets	0	0
Intercompany receivables	3,215	1,880
Current assets of discontinued operations	0	0
Other current assets	12	25
Total Current Assets	4,698	3,681
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Deferred tax assets	723	726
Intercompany receivables	261	261
Prepaid pension	5	5
Other non-current assets	120	119
Investment in subsidiary	15,836	17,137
Non-current assets of discontinued operations		0
TOTAL ASSETS	21,643	21,929
CURRENT LIABILITIES
Accounts payable and accrued liabilities	18	44
Short-term debt and current portion of long-term debt	60	50
Fiduciary liabilities	0	0
Intercompany payables	9,799	10,399
Current liabilities of discontinued operations	0	0
Other current liabilities	62	63
Total Current Liabilities	9,939	10,556
Long-term debt	1,413	1,413
Non-current deferred tax liabilities	0	0
Pension, other postretirement and postemployment liabilities	1,340	1,356
Intercompany payables	8,881	8,877
Other non-current liabilities	80	77
Non-current liabilities of discontinued operations		0
TOTAL LIABILITIES	21,653	22,279
TOTAL AON SHAREHOLDERS’ EQUITY	(10)	(350)
Noncontrolling interests	0	0
TOTAL EQUITY	(10)	(350)
TOTAL LIABILITIES AND EQUITY	21,643	21,929
Other Non-Guarantor Subsidiaries
CURRENT ASSETS :
Cash and cash equivalents	596	655
Short-term investments	154	150
Receivables, net	2,103	2,103
Fiduciary assets	9,162	8,959
Intercompany receivables	8,345	9,825
Current assets of discontinued operations	3,186	1,118
Other current assets	291	222
Total Current Assets	23,837	23,032
Goodwill	7,544	7,410
Intangible assets, net	1,886	1,890
Fixed assets, net	536	550
Deferred tax assets	172	171
Intercompany receivables	8,716	8,711
Prepaid pension	888	853
Other non-current assets	257	239
Investment in subsidiary	(10)	(350)
Non-current assets of discontinued operations		2,076
TOTAL ASSETS	43,826	44,582
CURRENT LIABILITIES
Accounts payable and accrued liabilities	2,065	2,837
Short-term debt and current portion of long-term debt	283	7
Fiduciary liabilities	9,162	8,959
Intercompany payables	1,667	1,269
Current liabilities of discontinued operations	1,036	940
Other current liabilities	711	593
Total Current Liabilities	14,924	14,605
Long-term debt	1	279
Non-current deferred tax liabilities	790	759
Pension, other postretirement and postemployment liabilities	391	404
Intercompany payables	468	461
Other non-current liabilities	637	634
Non-current liabilities of discontinued operations		139
TOTAL LIABILITIES	17,211	17,281
TOTAL AON SHAREHOLDERS’ EQUITY	26,543	27,244
Noncontrolling interests	72	57
TOTAL EQUITY	26,615	27,301
TOTAL LIABILITIES AND EQUITY	43,826	44,582
Elimination
CURRENT ASSETS :
Cash and cash equivalents	(1,588)	(1,862)
Short-term investments	0	0
Receivables, net	0	0
Fiduciary assets	0	0
Intercompany receivables	(11,633)	(11,810)
Current assets of discontinued operations	0	0
Other current assets	0	0
Total Current Assets	(13,221)	(13,672)
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Deferred tax assets	(678)	(706)
Intercompany receivables	(9,349)	(9,338)
Prepaid pension	0	0
Other non-current assets	0	0
Investment in subsidiary	(26,533)	(26,894)
Non-current assets of discontinued operations		0
TOTAL ASSETS	(49,781)	(50,610)
CURRENT LIABILITIES
Accounts payable and accrued liabilities	(1,588)	(1,862)
Short-term debt and current portion of long-term debt	0	0
Fiduciary liabilities	0	0
Intercompany payables	(11,633)	(11,810)
Current liabilities of discontinued operations	0	0
Other current liabilities	0	0
Total Current Liabilities	(13,221)	(13,672)
Long-term debt	0	0
Non-current deferred tax liabilities	(678)	(658)
Pension, other postretirement and postemployment liabilities	0	0
Intercompany payables	(9,349)	(9,338)
Other non-current liabilities	0	0
Non-current liabilities of discontinued operations		0
TOTAL LIABILITIES	(23,248)	(23,668)
TOTAL AON SHAREHOLDERS’ EQUITY	(26,533)	(26,942)
Noncontrolling interests	0	0
TOTAL EQUITY	(26,533)	(26,942)
TOTAL LIABILITIES AND EQUITY	$ (49,781)	$ (50,610)