Guarantee of Registered Securities - Condensed Consolidating Statement of Financial Position (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
CURRENT ASSETS :
Cash and cash equivalents	$ 684	$ 426
Short-term investments	2,746	290
Receivables, net	2,191	2,106
Fiduciary assets	9,582	8,959
Intercompany receivables	0	0
Other current assets	399	247
Current assets of discontinued operations	0	1,118
Total Current Assets	15,602	13,146
Goodwill	7,745	7,410
Intangible assets, net	1,402	1,890
Fixed assets, net	556	550
Deferred tax assets	575	325
Intercompany receivables	0	0
Prepaid pension	941	858
Other non-current assets	368	360
Investment in subsidiary	0	0
Non-current assets of discontinued operations	0	2,076
TOTAL ASSETS	27,189	26,615
CURRENT LIABILITIES
Accounts payable and accrued liabilities	1,423	1,604
Short-term debt and current portion of long-term debt	292	336
Fiduciary liabilities	9,582	8,959
Intercompany payables	0	0
Current liabilities of discontinued operations	0	940
Other current liabilities	2,078	656
Total Current Liabilities	13,375	12,495
Long-term debt	5,631	5,869
Deferred tax liabilities	84	101
Pension, other postretirement and postemployment liabilities	1,688	1,760
Intercompany payables	0	0
Other non-current liabilities	858	719
Non-current liabilities of discontinued operations	0	139
TOTAL LIABILITIES	21,636	21,083
TOTAL AON SHAREHOLDERS’ EQUITY	5,487	5,475
Noncontrolling interests	66	57
TOTAL EQUITY	5,553	5,532
TOTAL LIABILITIES AND EQUITY	27,189	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	106	105
Other current assets	0	0
Current assets of discontinued operations	0	0
Total Current Assets	106	105
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Deferred tax assets	134	134
Intercompany receivables	380	366
Prepaid pension	0	0
Other non-current assets	2	2
Investment in subsidiary	11,677	10,107
Non-current assets of discontinued operations	0	0
TOTAL ASSETS	12,299	10,714
CURRENT LIABILITIES
Accounts payable and accrued liabilities	2,336	585
Short-term debt and current portion of long-term debt	0	279
Fiduciary liabilities	0	0
Intercompany payables	186	142
Current liabilities of discontinued operations	0	0
Other current liabilities	60	0
Total Current Liabilities	2,582	1,006
Long-term debt	4,216	4,177
Deferred tax liabilities	0	0
Pension, other postretirement and postemployment liabilities	0	0
Intercompany payables	0	0
Other non-current liabilities	14	8
Non-current liabilities of discontinued operations	0	0
TOTAL LIABILITIES	6,812	5,191
TOTAL AON SHAREHOLDERS’ EQUITY	5,487	5,523
Noncontrolling interests	0	0
TOTAL EQUITY	5,487	5,523
TOTAL LIABILITIES AND EQUITY	12,299	10,714
Aon Corporation
CURRENT ASSETS :
Cash and cash equivalents	2,675	1,633
Short-term investments	2,580	140
Receivables, net	4	3
Fiduciary assets	0	0
Intercompany receivables	4,176	1,880
Other current assets	37	25
Current assets of discontinued operations	0	0
Total Current Assets	9,472	3,681
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Deferred tax assets	676	726
Intercompany receivables	261	261
Prepaid pension	5	5
Other non-current assets	121	119
Investment in subsidiary	16,596	17,131
Non-current assets of discontinued operations	0	0
TOTAL ASSETS	27,131	21,923
CURRENT LIABILITIES
Accounts payable and accrued liabilities	38	44
Short-term debt and current portion of long-term debt	2	50
Fiduciary liabilities	0	0
Intercompany payables	14,770	10,399
Current liabilities of discontinued operations	0	0
Other current liabilities	54	63
Total Current Liabilities	14,864	10,556
Long-term debt	1,414	1,413
Deferred tax liabilities	0	0
Pension, other postretirement and postemployment liabilities	1,307	1,356
Intercompany payables	8,895	8,877
Other non-current liabilities	119	77
Non-current liabilities of discontinued operations	0	0
TOTAL LIABILITIES	26,599	22,279
TOTAL AON SHAREHOLDERS’ EQUITY	532	(356)
Noncontrolling interests	0	0
TOTAL EQUITY	532	(356)
TOTAL LIABILITIES AND EQUITY	27,131	21,923
Other Non-Guarantor Subsidiaries
CURRENT ASSETS :
Cash and cash equivalents	801	655
Short-term investments	166	150
Receivables, net	2,187	2,103
Fiduciary assets	9,582	8,959
Intercompany receivables	12,476	9,825
Other current assets	362	222
Current assets of discontinued operations	0	1,118
Total Current Assets	25,574	23,032
Goodwill	7,745	7,410
Intangible assets, net	1,402	1,890
Fixed assets, net	556	550
Deferred tax assets	169	171
Intercompany receivables	8,729	8,711
Prepaid pension	936	853
Other non-current assets	245	239
Investment in subsidiary	532	(356)
Non-current assets of discontinued operations	0	2,076
TOTAL ASSETS	45,888	44,576
CURRENT LIABILITIES
Accounts payable and accrued liabilities	1,841	2,837
Short-term debt and current portion of long-term debt	290	7
Fiduciary liabilities	9,582	8,959
Intercompany payables	1,802	1,269
Current liabilities of discontinued operations	0	940
Other current liabilities	1,964	593
Total Current Liabilities	15,479	14,605
Long-term debt	1	279
Deferred tax liabilities	488	759
Pension, other postretirement and postemployment liabilities	381	404
Intercompany payables	475	461
Other non-current liabilities	725	634
Non-current liabilities of discontinued operations	0	139
TOTAL LIABILITIES	17,549	17,281
TOTAL AON SHAREHOLDERS’ EQUITY	28,273	27,238
Noncontrolling interests	66	57
TOTAL EQUITY	28,339	27,295
TOTAL LIABILITIES AND EQUITY	45,888	44,576
Consolidation Adjustments
CURRENT ASSETS :
Cash and cash equivalents	(2,792)	(1,862)
Short-term investments	0	0
Receivables, net	0	0
Fiduciary assets	0	0
Intercompany receivables	(16,758)	(11,810)
Other current assets	0	0
Current assets of discontinued operations	0	0
Total Current Assets	(19,550)	(13,672)
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Deferred tax assets	(404)	(706)
Intercompany receivables	(9,370)	(9,338)
Prepaid pension	0	0
Other non-current assets	0	0
Investment in subsidiary	(28,805)	(26,882)
Non-current assets of discontinued operations	0	0
TOTAL ASSETS	(58,129)	(50,598)
CURRENT LIABILITIES
Accounts payable and accrued liabilities	(2,792)	(1,862)
Short-term debt and current portion of long-term debt	0	0
Fiduciary liabilities	0	0
Intercompany payables	(16,758)	(11,810)
Current liabilities of discontinued operations	0	0
Other current liabilities	0	0
Total Current Liabilities	(19,550)	(13,672)
Long-term debt	0	0
Deferred tax liabilities	(404)	(658)
Pension, other postretirement and postemployment liabilities	0	0
Intercompany payables	(9,370)	(9,338)
Other non-current liabilities	0	0
Non-current liabilities of discontinued operations	0	0
TOTAL LIABILITIES	(29,324)	(23,668)
TOTAL AON SHAREHOLDERS’ EQUITY	(28,805)	(26,930)
Noncontrolling interests	0	0
TOTAL EQUITY	(28,805)	(26,930)
TOTAL LIABILITIES AND EQUITY	$ (58,129)	$ (50,598)