Guarantee of Registered Securities - Condensed Consolidating Statement of Financial Position (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Total Current Assets
Cash and cash equivalents	$ 756	$ 426
Short-term investments	529	290
Receivables, net	2,478	2,106
Fiduciary assets	9,625	8,959
Intercompany receivables	0	0
Other current assets	289	247
Current assets of discontinued operations	0	1,118
Total Current Assets	13,677	13,146
Goodwill	8,358	7,410	$ 7,068
Intangible assets, net	1,733	1,890
Fixed assets, net	564	550
Deferred tax assets	389	325
Deferred tax asset	0	0
Prepaid pension	1,060	858
Other non-current assets	307	360
Investment in subsidiary	0	0
Non-current assets of discontinued operations	0	2,076
TOTAL ASSETS	26,088	26,615
CURRENT LIABILITIES
Accounts payable and accrued liabilities	1,961	1,604
Short-term debt and current portion of long-term debt	299	336
Fiduciary liabilities	9,625	8,959
Intercompany payables	0	0
Other current liabilities	870	656
Current liabilities of discontinued operations	0	940
Total Current Liabilities	12,755	12,495
Long-term debt	5,667	5,869
Deferred tax liabilities	127	101
Pension, other postretirement and other post-employment liabilities	1,789	1,760
Intercompany payables	0	0
Other non-current liabilities	1,102	719
Non-current liabilities of discontinued operations	0	139
TOTAL LIABILITIES	21,440	21,083
TOTAL AON SHAREHOLDERS’ EQUITY	4,583	5,475
Noncontrolling interests	65	57
TOTAL EQUITY	4,648	5,532	$ 6,059	$ 6,527
TOTAL LIABILITIES AND EQUITY	26,088	26,615
Aon plc
Total Current Assets
Cash and cash equivalents	1	0
Short-term investments	0	0
Receivables, net	0	0
Fiduciary assets	0	0
Intercompany receivables	165	105
Other current assets	1	0
Current assets of discontinued operations	0	0
Total Current Assets	167	105
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Deferred tax assets	99	134
Deferred tax asset	414	366
Prepaid pension	0	0
Other non-current assets	1	2
Investment in subsidiary	8,884	10,107
Non-current assets of discontinued operations	0	0
TOTAL ASSETS	9,565	10,714
CURRENT LIABILITIES
Accounts payable and accrued liabilities	574	585
Short-term debt and current portion of long-term debt	0	279
Fiduciary liabilities	0	0
Intercompany payables	130	142
Other current liabilities	16	0
Current liabilities of discontinued operations	0	0
Total Current Liabilities	720	1,006
Long-term debt	4,251	4,177
Deferred tax liabilities	0	0
Pension, other postretirement and other post-employment liabilities	0	0
Intercompany payables	0	0
Other non-current liabilities	11	8
Non-current liabilities of discontinued operations	0	0
TOTAL LIABILITIES	4,982	5,191
TOTAL AON SHAREHOLDERS’ EQUITY	4,583	5,523
Noncontrolling interests	0	0
TOTAL EQUITY	4,583	5,523
TOTAL LIABILITIES AND EQUITY	9,565	10,714
Aon Corporation
Total Current Assets
Cash and cash equivalents	2,524	1,633
Short-term investments	355	140
Receivables, net	2	3
Fiduciary assets	0	0
Intercompany receivables	1,046	1,880
Other current assets	29	25
Current assets of discontinued operations	0	0
Total Current Assets	3,956	3,681
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Deferred tax assets	396	726
Deferred tax asset	261	261
Prepaid pension	6	5
Other non-current assets	35	119
Investment in subsidiary	17,910	17,131
Non-current assets of discontinued operations	0	0
TOTAL ASSETS	22,564	21,923
CURRENT LIABILITIES
Accounts payable and accrued liabilities	36	44
Short-term debt and current portion of long-term debt	0	50
Fiduciary liabilities	0	0
Intercompany payables	11,149	10,399
Other current liabilities	64	63
Current liabilities of discontinued operations	0	0
Total Current Liabilities	11,249	10,556
Long-term debt	1,415	1,413
Deferred tax liabilities	0	0
Pension, other postretirement and other post-employment liabilities	1,391	1,356
Intercompany payables	8,398	8,877
Other non-current liabilities	91	77
Non-current liabilities of discontinued operations	0	0
TOTAL LIABILITIES	22,544	22,279
TOTAL AON SHAREHOLDERS’ EQUITY	20	(356)
Noncontrolling interests	0	0
TOTAL EQUITY	20	(356)
TOTAL LIABILITIES AND EQUITY	22,564	21,923
Other Non-Guarantor Subsidiaries
Total Current Assets
Cash and cash equivalents	793	655
Short-term investments	174	150
Receivables, net	2,476	2,103
Fiduciary assets	9,625	8,959
Intercompany receivables	10,824	9,825
Other current assets	259	222
Current assets of discontinued operations	0	1,118
Total Current Assets	24,151	23,032
Goodwill	8,358	7,410
Intangible assets, net	1,733	1,890
Fixed assets, net	564	550
Deferred tax assets	143	171
Deferred tax asset	8,232	8,711
Prepaid pension	1,054	853
Other non-current assets	271	239
Investment in subsidiary	20	(356)
Non-current assets of discontinued operations	0	2,076
TOTAL ASSETS	44,526	44,576
CURRENT LIABILITIES
Accounts payable and accrued liabilities	3,913	2,837
Short-term debt and current portion of long-term debt	299	7
Fiduciary liabilities	9,625	8,959
Intercompany payables	756	1,269
Other current liabilities	790	593
Current liabilities of discontinued operations	0	940
Total Current Liabilities	15,383	14,605
Long-term debt	1	279
Deferred tax liabilities	376	759
Pension, other postretirement and other post-employment liabilities	398	404
Intercompany payables	509	461
Other non-current liabilities	1,000	634
Non-current liabilities of discontinued operations	0	139
TOTAL LIABILITIES	17,667	17,281
TOTAL AON SHAREHOLDERS’ EQUITY	26,794	27,238
Noncontrolling interests	65	57
TOTAL EQUITY	26,859	27,295
TOTAL LIABILITIES AND EQUITY	44,526	44,576
Consolidating Adjustments
Total Current Assets
Cash and cash equivalents	(2,562)	(1,862)
Short-term investments	0	0
Receivables, net	0	0
Fiduciary assets	0	0
Intercompany receivables	(12,035)	(11,810)
Other current assets	0	0
Current assets of discontinued operations	0	0
Total Current Assets	(14,597)	(13,672)
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Deferred tax assets	(249)	(706)
Deferred tax asset	(8,907)	(9,338)
Prepaid pension	0	0
Other non-current assets	0	0
Investment in subsidiary	(26,814)	(26,882)
Non-current assets of discontinued operations	0	0
TOTAL ASSETS	(50,567)	(50,598)
CURRENT LIABILITIES
Accounts payable and accrued liabilities	(2,562)	(1,862)
Short-term debt and current portion of long-term debt	0	0
Fiduciary liabilities	0	0
Intercompany payables	(12,035)	(11,810)
Other current liabilities	0	0
Current liabilities of discontinued operations	0	0
Total Current Liabilities	(14,597)	(13,672)
Long-term debt	0	0
Deferred tax liabilities	(249)	(658)
Pension, other postretirement and other post-employment liabilities	0	0
Intercompany payables	(8,907)	(9,338)
Other non-current liabilities	0	0
Non-current liabilities of discontinued operations	0	0
TOTAL LIABILITIES	(23,753)	(23,668)
TOTAL AON SHAREHOLDERS’ EQUITY	(26,814)	(26,930)
Noncontrolling interests	0	0
TOTAL EQUITY	(26,814)	(26,930)
TOTAL LIABILITIES AND EQUITY	$ (50,567)	$ (50,598)