Guarantee of Registered Securities - Condensed Consolidating Statement of Financial Position (Details) - USD ($) $ in Millions	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Current assets
Cash and cash equivalents	$ 581		$ 656
Short-term investments	235		172
Receivables, net	3,227		2,760
Fiduciary assets	12,071		10,166
Current intercompany receivables	0		0
Other current assets	631		618
Total current assets	16,745		14,372
Goodwill	8,198		8,171
Intangible assets, net	973		1,149
Fixed assets, net	599		588
Operating lease right-of-use assets	959
Deferred tax assets	599		561
Prepaid pension	1,213		1,133
Non-current intercompany receivables	0		0
Other non-current assets	521		448
Investment in subsidiary	0		0
Total assets	29,807		26,422
Current liabilities
Accounts payable and accrued liabilities	1,369		1,943
Short-term debt and current portion of long-term debt	844		251
Fiduciary liabilities	12,071		10,166
Current intercompany payables	0		0
Other current liabilities	1,197		936
Total current liabilities	15,481		13,296
Long-term debt	6,740		5,993
Non-current operating lease liabilities	962
Deferred tax liabilities	211		181
Pension, other postretirement, and postemployment liabilities	1,576		1,636
Non-current intercompany payables	0		0
Other non-current liabilities	924		1,097
Total liabilities	25,894		22,203
Total Aon shareholders’ equity	3,836		4,151
Noncontrolling interests	77		68
Total equity	3,913	$ 4,862	4,219	$ 4,618	$ 5,385	$ 5,140
Total liabilities and equity	29,807		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	167		191
Other current assets	0		0
Total current assets	167		191
Goodwill	0		0
Intangible assets, net	0		0
Fixed assets, net	0		0
Operating lease right-of-use assets	0
Deferred tax assets	93		94
Prepaid pension	0		0
Non-current intercompany receivables	404		403
Other non-current assets	1		1
Investment in subsidiary	8,937		8,433
Total assets	9,602		9,122
Current liabilities
Accounts payable and accrued liabilities	582		274
Short-term debt and current portion of long-term debt	569		250
Fiduciary liabilities	0		0
Current intercompany payables	378		213
Other current liabilities	0		0
Total current liabilities	1,529		737
Long-term debt	4,234		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	3		3
Total liabilities	5,766		4,971
Total Aon shareholders’ equity	3,836		4,151
Noncontrolling interests	0		0
Total equity	3,836		4,151
Total liabilities and equity	9,602		9,122
Aon Corporation
Current assets
Cash and cash equivalents	1,371		862
Short-term investments	125		56
Receivables, net	0		0
Fiduciary assets	0		0
Current intercompany receivables	3,421		897
Other current assets	10		16
Total current assets	4,927		1,831
Goodwill	0		0
Intangible assets, net	0		0
Fixed assets, net	0		0
Operating lease right-of-use assets	112
Deferred tax assets	498		467
Prepaid pension	5		5
Non-current intercompany receivables	261		261
Other non-current assets	28		30
Investment in subsidiary	20,147		19,132
Total assets	25,978		21,726
Current liabilities
Accounts payable and accrued liabilities	57		70
Short-term debt and current portion of long-term debt	275		0
Fiduciary liabilities	0		0
Current intercompany payables	14,633		11,875
Other current liabilities	78		69
Total current liabilities	15,043		12,014
Long-term debt	2,506		1,762
Non-current operating lease liabilities	149
Deferred tax liabilities	0		0
Pension, other postretirement, and postemployment liabilities	1,232		1,275
Non-current intercompany payables	7,366		7,390
Other non-current liabilities	106		167
Total liabilities	26,402		22,608
Total Aon shareholders’ equity	(424)		(882)
Noncontrolling interests	0		0
Total equity	(424)		(882)
Total liabilities and equity	25,978		21,726
Other Non-Guarantor Subsidiaries
Current assets
Cash and cash equivalents	537		575
Short-term investments	110		116
Receivables, net	3,227		2,760
Fiduciary assets	12,071		10,166
Current intercompany receivables	12,843		11,634
Other current assets	621		602
Total current assets	29,409		25,853
Goodwill	8,198		8,171
Intangible assets, net	973		1,149
Fixed assets, net	599		588
Operating lease right-of-use assets	847
Deferred tax assets	150		144
Prepaid pension	1,208		1,128
Non-current intercompany receivables	7,200		7,225
Other non-current assets	492		417
Investment in subsidiary	(424)		(882)
Total assets	48,652		43,793
Current liabilities
Accounts payable and accrued liabilities	2,057		2,380
Short-term debt and current portion of long-term debt	0		1
Fiduciary liabilities	12,071		10,166
Current intercompany payables	1,420		634
Other current liabilities	1,119		867
Total current liabilities	16,667		14,048
Long-term debt	0		0
Non-current operating lease liabilities	813
Deferred tax liabilities	353		325
Pension, other postretirement, and postemployment liabilities	344		361
Non-current intercompany payables	499		499
Other non-current liabilities	815		927
Total liabilities	19,491		16,160
Total Aon shareholders’ equity	29,084		27,565
Noncontrolling interests	77		68
Total equity	29,161		27,633
Total liabilities and equity	48,652		43,793
Consolidation Adjustments
Current assets
Cash and cash equivalents	(1,327)		(781)
Short-term investments	0		0
Receivables, net	0		0
Fiduciary assets	0		0
Current intercompany receivables	(16,431)		(12,722)
Other current assets	0		0
Total current assets	(17,758)		(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	(142)		(144)
Prepaid pension	0		0
Non-current intercompany receivables	(7,865)		(7,889)
Other non-current assets	0		0
Investment in subsidiary	(28,660)		(26,683)
Total assets	(54,425)		(48,219)
Current liabilities
Accounts payable and accrued liabilities	(1,327)		(781)
Short-term debt and current portion of long-term debt	0		0
Fiduciary liabilities	0		0
Current intercompany payables	(16,431)		(12,722)
Other current liabilities	0		0
Total current liabilities	(17,758)		(13,503)
Long-term debt	0		0
Non-current operating lease liabilities	0
Deferred tax liabilities	(142)		(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	(25,765)		(21,536)
Total Aon shareholders’ equity	(28,660)		(26,683)
Noncontrolling interests	0		0
Total equity	(28,660)		(26,683)
Total liabilities and equity	$ (54,425)		$ (48,219)