Guarantee of Registered Securities - Condensed Consolidating Statement of Financial Position (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Current assets
Cash and cash equivalents	$ 600	$ 656
Short-term investments	134	172
Receivables, net	3,242	2,760
Fiduciary assets	11,412	10,166
Current intercompany receivables	0	0
Other current assets	531	618
Total current assets	15,919	14,372
Goodwill	8,219	8,171
Intangible assets, net	1,077	1,149
Fixed assets, net	606	588
Operating lease right-of-use assets	993
Deferred tax assets	588	561
Prepaid pension	1,224	1,133
Non-current intercompany receivables	0	0
Other non-current assets	509	448
Investment in subsidiary	0	0
Total assets	29,135	26,422
Current liabilities
Accounts payable and accrued liabilities	1,479	1,943
Short-term debt and current portion of long-term debt	426	251
Fiduciary liabilities	11,412	10,166
Current intercompany payables	0	0
Other current liabilities	1,220	936
Total current liabilities	14,537	13,296
Long-term debt	5,990	5,993
Non-current operating lease liabilities	978
Deferred tax liabilities	205	181
Pension, other postretirement, and postemployment liabilities	1,590	1,636
Non-current intercompany payables	0	0
Other non-current liabilities	973	1,097
Total liabilities	24,273	22,203
Total Aon shareholders’ equity	4,775	4,151
Noncontrolling interests	87	68
Total equity	4,862	4,219	$ 5,385	$ 5,140
Total liabilities and equity	29,135	26,422
Aon plc
Current assets
Cash and cash equivalents	0	0
Short-term investments	0	0
Receivables, net	0	0
Fiduciary assets	0	0
Current intercompany receivables	166	191
Other current assets	0	0
Total current assets	166	191
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Operating lease right-of-use assets	0
Deferred tax assets	94	94
Prepaid pension	0	0
Non-current intercompany receivables	401	403
Other non-current assets	1	1
Investment in subsidiary	9,283	8,433
Total assets	9,945	9,122
Current liabilities
Accounts payable and accrued liabilities	351	274
Short-term debt and current portion of long-term debt	248	250
Fiduciary liabilities	0	0
Current intercompany payables	339	213
Other current liabilities	0	0
Total current liabilities	938	737
Long-term debt	4,228	4,231
Non-current operating lease liabilities	0
Deferred tax liabilities	0	0
Pension, other postretirement, and postemployment liabilities	0	0
Non-current intercompany payables	0	0
Other non-current liabilities	4	3
Total liabilities	5,170	4,971
Total Aon shareholders’ equity	4,775	4,151
Noncontrolling interests	0	0
Total equity	4,775	4,151
Total liabilities and equity	9,945	9,122
Aon Corporation
Current assets
Cash and cash equivalents	746	862
Short-term investments	47	56
Receivables, net	0	0
Fiduciary assets	0	0
Current intercompany receivables	2,277	897
Other current assets	10	16
Total current assets	3,080	1,831
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Operating lease right-of-use assets	114
Deferred tax assets	488	467
Prepaid pension	5	5
Non-current intercompany receivables	262	261
Other non-current assets	30	30
Investment in subsidiary	19,919	19,132
Total assets	23,898	21,726
Current liabilities
Accounts payable and accrued liabilities	75	70
Short-term debt and current portion of long-term debt	175	0
Fiduciary liabilities	0	0
Current intercompany payables	13,283	11,875
Other current liabilities	81	69
Total current liabilities	13,614	12,014
Long-term debt	1,762	1,762
Non-current operating lease liabilities	153
Deferred tax liabilities	0	0
Pension, other postretirement, and postemployment liabilities	1,241	1,275
Non-current intercompany payables	7,368	7,390
Other non-current liabilities	115	167
Total liabilities	24,253	22,608
Total Aon shareholders’ equity	(355)	(882)
Noncontrolling interests	0	0
Total equity	(355)	(882)
Total liabilities and equity	23,898	21,726
Other Non-Guarantor Subsidiaries
Current assets
Cash and cash equivalents	563	575
Short-term investments	87	116
Receivables, net	3,242	2,760
Fiduciary assets	11,412	10,166
Current intercompany receivables	12,154	11,634
Other current assets	521	602
Total current assets	27,979	25,853
Goodwill	8,219	8,171
Intangible assets, net	1,077	1,149
Fixed assets, net	606	588
Operating lease right-of-use assets	879
Deferred tax assets	150	144
Prepaid pension	1,219	1,128
Non-current intercompany receivables	7,202	7,225
Other non-current assets	478	417
Investment in subsidiary	(355)	(882)
Total assets	47,454	43,793
Current liabilities
Accounts payable and accrued liabilities	1,762	2,380
Short-term debt and current portion of long-term debt	3	1
Fiduciary liabilities	11,412	10,166
Current intercompany payables	975	634
Other current liabilities	1,139	867
Total current liabilities	15,291	14,048
Long-term debt	0	0
Non-current operating lease liabilities	825
Deferred tax liabilities	349	325
Pension, other postretirement, and postemployment liabilities	349	361
Non-current intercompany payables	497	499
Other non-current liabilities	854	927
Total liabilities	18,165	16,160
Total Aon shareholders’ equity	29,202	27,565
Noncontrolling interests	87	68
Total equity	29,289	27,633
Total liabilities and equity	47,454	43,793
Consolidation Adjustments
Current assets
Cash and cash equivalents	(709)	(781)
Short-term investments	0	0
Receivables, net	0	0
Fiduciary assets	0	0
Current intercompany receivables	(14,597)	(12,722)
Other current assets	0	0
Total current assets	(15,306)	(13,503)
Goodwill	0	0
Intangible assets, net	0	0
Fixed assets, net	0	0
Operating lease right-of-use assets	0
Deferred tax assets	(144)	(144)
Prepaid pension	0	0
Non-current intercompany receivables	(7,865)	(7,889)
Other non-current assets	0	0
Investment in subsidiary	(28,847)	(26,683)
Total assets	(52,162)	(48,219)
Current liabilities
Accounts payable and accrued liabilities	(709)	(781)
Short-term debt and current portion of long-term debt	0	0
Fiduciary liabilities	0	0
Current intercompany payables	(14,597)	(12,722)
Other current liabilities	0	0
Total current liabilities	(15,306)	(13,503)
Long-term debt	0	0
Non-current operating lease liabilities	0
Deferred tax liabilities	(144)	(144)
Pension, other postretirement, and postemployment liabilities	0	0
Non-current intercompany payables	(7,865)	(7,889)
Other non-current liabilities	0	0
Total liabilities	(23,315)	(21,536)
Total Aon shareholders’ equity	(28,847)	(26,683)
Noncontrolling interests	0	0
Total equity	(28,847)	(26,683)
Total liabilities and equity	$ (52,162)	$ (48,219)