Guarantee of Registered Securities - Condensed Consolidating Statement of Financial Position (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
CURRENT ASSETS :
Cash and cash equivalents	$ 749	$ 426
Short-term investments	1,640	290
Receivables, net	2,068	2,106
Fiduciary assets	9,292	8,959
Intercompany receivables	0	0
Other current assets	518	247
Current assets of discontinued operations	0	1,118
Total Current Assets	14,267	13,146
Goodwill	7,888	7,410
Intangible assets, net	1,341	1,890
Fixed assets, net	545	550
Deferred tax assets	565	325
Intercompany receivables	0	0
Prepaid pension	1,020	858
Other non-current assets	298	360
Investment in subsidiary	0	0
Non-current assets of discontinued operations	0	2,076
TOTAL ASSETS	25,924	26,615
CURRENT LIABILITIES
Accounts payable and accrued liabilities	1,588	1,604
Short-term debt and current portion of long-term debt	305	336
Fiduciary liabilities	9,292	8,959
Intercompany payables	0	0
Other current liabilities	1,289	656
Current liabilities of discontinued operations	0	940
Total Current Liabilities	12,474	12,495
Long-term debt	5,662	5,869
Deferred tax liabilities	83	101
Pension, other postretirement and postemployment liabilities	1,612	1,760
Intercompany payables	0	0
Other non-current liabilities	846	719
Non-current liabilities of discontinued operations	0	139
TOTAL LIABILITIES	20,677	21,083
TOTAL AON SHAREHOLDERS’ EQUITY	5,175	5,475
Noncontrolling interests	72	57
TOTAL EQUITY	5,247	5,532
TOTAL LIABILITIES AND EQUITY	25,924	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	110	105
Other current assets	0	0
Current assets of discontinued operations	0	0
Total Current Assets	110	105
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Deferred tax assets	135	134
Intercompany receivables	391	366
Prepaid pension	0	0
Other non-current assets	1	2
Investment in subsidiary	11,900	10,107
Non-current assets of discontinued operations	0	0
TOTAL ASSETS	12,537	10,714
CURRENT LIABILITIES
Accounts payable and accrued liabilities	2,929	585
Short-term debt and current portion of long-term debt	0	279
Fiduciary liabilities	0	0
Intercompany payables	147	142
Other current liabilities	24	0
Current liabilities of discontinued operations	0	0
Total Current Liabilities	3,100	1,006
Long-term debt	4,247	4,177
Deferred tax liabilities	0	0
Pension, other postretirement and postemployment liabilities	0	0
Intercompany payables	0	0
Other non-current liabilities	15	8
Non-current liabilities of discontinued operations	0	0
TOTAL LIABILITIES	7,362	5,191
TOTAL AON SHAREHOLDERS’ EQUITY	5,175	5,523
Noncontrolling interests	0	0
TOTAL EQUITY	5,175	5,523
TOTAL LIABILITIES AND EQUITY	12,537	10,714
Aon Corporation
CURRENT ASSETS :
Cash and cash equivalents	3,110	1,633
Short-term investments	1,467	140
Receivables, net	2	3
Fiduciary assets	0	0
Intercompany receivables	4,860	1,880
Other current assets	37	25
Current assets of discontinued operations	0	0
Total Current Assets	9,476	3,681
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Deferred tax assets	664	726
Intercompany receivables	261	261
Prepaid pension	5	5
Other non-current assets	49	119
Investment in subsidiary	17,748	17,131
Non-current assets of discontinued operations	0	0
TOTAL ASSETS	28,203	21,923
CURRENT LIABILITIES
Accounts payable and accrued liabilities	37	44
Short-term debt and current portion of long-term debt	0	50
Fiduciary liabilities	0	0
Intercompany payables	15,951	10,399
Other current liabilities	54	63
Current liabilities of discontinued operations	0	0
Total Current Liabilities	16,042	10,556
Long-term debt	1,414	1,413
Deferred tax liabilities	0	0
Pension, other postretirement and postemployment liabilities	1,234	1,356
Intercompany payables	8,894	8,877
Other non-current liabilities	110	77
Non-current liabilities of discontinued operations	0	0
TOTAL LIABILITIES	27,694	22,279
TOTAL AON SHAREHOLDERS’ EQUITY	509	(356)
Noncontrolling interests	0	0
TOTAL EQUITY	509	(356)
TOTAL LIABILITIES AND EQUITY	28,203	21,923
Other Non-Guarantor Subsidiaries
CURRENT ASSETS :
Cash and cash equivalents	802	655
Short-term investments	173	150
Receivables, net	2,066	2,103
Fiduciary assets	9,292	8,959
Intercompany receivables	12,436	9,825
Other current assets	481	222
Current assets of discontinued operations	0	1,118
Total Current Assets	25,250	23,032
Goodwill	7,888	7,410
Intangible assets, net	1,341	1,890
Fixed assets, net	545	550
Deferred tax assets	173	171
Intercompany receivables	8,728	8,711
Prepaid pension	1,015	853
Other non-current assets	248	239
Investment in subsidiary	509	(356)
Non-current assets of discontinued operations	0	2,076
TOTAL ASSETS	45,697	44,576
CURRENT LIABILITIES
Accounts payable and accrued liabilities	1,785	2,837
Short-term debt and current portion of long-term debt	305	7
Fiduciary liabilities	9,292	8,959
Intercompany payables	1,308	1,269
Other current liabilities	1,211	593
Current liabilities of discontinued operations	0	940
Total Current Liabilities	13,901	14,605
Long-term debt	1	279
Deferred tax liabilities	490	759
Pension, other postretirement and postemployment liabilities	378	404
Intercompany payables	486	461
Other non-current liabilities	721	634
Non-current liabilities of discontinued operations	0	139
TOTAL LIABILITIES	15,977	17,281
TOTAL AON SHAREHOLDERS’ EQUITY	29,648	27,238
Noncontrolling interests	72	57
TOTAL EQUITY	29,720	27,295
TOTAL LIABILITIES AND EQUITY	45,697	44,576
Consolidation Adjustments
CURRENT ASSETS :
Cash and cash equivalents	(3,163)	(1,862)
Short-term investments	0	0
Receivables, net	0	0
Fiduciary assets	0	0
Intercompany receivables	(17,406)	(11,810)
Other current assets	0	0
Current assets of discontinued operations	0	0
Total Current Assets	(20,569)	(13,672)
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Deferred tax assets	(407)	(706)
Intercompany receivables	(9,380)	(9,338)
Prepaid pension	0	0
Other non-current assets	0	0
Investment in subsidiary	(30,157)	(26,882)
Non-current assets of discontinued operations	0	0
TOTAL ASSETS	(60,513)	(50,598)
CURRENT LIABILITIES
Accounts payable and accrued liabilities	(3,163)	(1,862)
Short-term debt and current portion of long-term debt	0	0
Fiduciary liabilities	0	0
Intercompany payables	(17,406)	(11,810)
Other current liabilities	0	0
Current liabilities of discontinued operations	0	0
Total Current Liabilities	(20,569)	(13,672)
Long-term debt	0	0
Deferred tax liabilities	(407)	(658)
Pension, other postretirement and postemployment liabilities	0	0
Intercompany payables	(9,380)	(9,338)
Other non-current liabilities	0	0
Non-current liabilities of discontinued operations	0	0
TOTAL LIABILITIES	(30,356)	(23,668)
TOTAL AON SHAREHOLDERS’ EQUITY	(30,157)	(26,930)
Noncontrolling interests	0	0
TOTAL EQUITY	(30,157)	(26,930)
TOTAL LIABILITIES AND EQUITY	$ (60,513)	$ (50,598)