Guarantee of Registered Securities - Condensed Consolidating Statement of Financial Position (Details) - USD ($) $ in Millions	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Current assets
Cash and cash equivalents	$ 602			$ 656
Short-term investments	177			172
Receivables, net	2,866			2,760
Fiduciary assets	11,041			10,166
Current intercompany receivables	0			0
Other current assets	631			618
Total current assets	15,317			14,372
Goodwill	8,071			8,171
Intangible assets, net	874			1,149
Fixed assets, net	606			588
Operating lease right-of-use assets	913
Deferred tax assets	611			561
Prepaid pension	1,213			1,133
Non-current intercompany receivables	0			0
Other non-current assets	562			448
Investment in subsidiary	0			0
Total assets	28,167			26,422
Current liabilities
Accounts payable and accrued liabilities	1,499			1,943
Short-term debt and current portion of long-term debt	1,148			251
Fiduciary liabilities	11,041			10,166
Current intercompany payables	0			0
Other current liabilities	1,163			936
Total current liabilities	14,851			13,296
Long-term debt	6,120			5,993
Non-current operating lease liabilities	922
Deferred tax liabilities	209			181
Pension, other postretirement, and postemployment liabilities	1,551			1,636
Non-current intercompany payables	0			0
Other non-current liabilities	953			1,097
Total liabilities	24,606			22,203
Total Aon shareholders’ equity	3,492			4,151
Noncontrolling interests	69			68
Total equity	3,561	$ 3,913	$ 4,862	4,219	$ 4,328	$ 4,618	$ 5,385	$ 5,140
Total liabilities and equity	28,167			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	153			191
Other current assets	0			0
Total current assets	153			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	394			403
Other non-current assets	2			1
Investment in subsidiary	9,052			8,433
Total assets	9,694			9,122
Current liabilities
Accounts payable and accrued liabilities	1,171			274
Short-term debt and current portion of long-term debt	546			250
Fiduciary liabilities	0			0
Current intercompany payables	268			213
Other current liabilities	0			0
Total current liabilities	1,985			737
Long-term debt	4,213			4,231
Non-current operating lease liabilities	0
Deferred tax liabilities	1			0
Pension, other postretirement, and postemployment liabilities	0			0
Non-current intercompany payables	0			0
Other non-current liabilities	3			3
Total liabilities	6,202			4,971
Total Aon shareholders’ equity	3,492			4,151
Noncontrolling interests	0			0
Total equity	3,492			4,151
Total liabilities and equity	9,694			9,122
Aon Corporation
Current assets
Cash and cash equivalents	1,530			862
Short-term investments	63			56
Receivables, net	0			0
Fiduciary assets	0			0
Current intercompany receivables	4,090			897
Other current assets	6			16
Total current assets	5,689			1,831
Goodwill	0			0
Intangible assets, net	0			0
Fixed assets, net	0			0
Operating lease right-of-use assets	109
Deferred tax assets	513			467
Prepaid pension	5			5
Non-current intercompany receivables	261			261
Other non-current assets	35			30
Investment in subsidiary	20,085			19,132
Total assets	26,697			21,726
Current liabilities
Accounts payable and accrued liabilities	42			70
Short-term debt and current portion of long-term debt	599			0
Fiduciary liabilities	0			0
Current intercompany payables	15,956			11,875
Other current liabilities	79			69
Total current liabilities	16,676			12,014
Long-term debt	1,907			1,762
Non-current operating lease liabilities	146
Deferred tax liabilities	0			0
Pension, other postretirement, and postemployment liabilities	1,222			1,275
Non-current intercompany payables	7,364			7,390
Other non-current liabilities	107			167
Total liabilities	27,422			22,608
Total Aon shareholders’ equity	(725)			(882)
Noncontrolling interests	0			0
Total equity	(725)			(882)
Total liabilities and equity	26,697			21,726
Other Non-Guarantor Subsidiaries
Current assets
Cash and cash equivalents	491			575
Short-term investments	114			116
Receivables, net	2,866			2,760
Fiduciary assets	11,041			10,166
Current intercompany receivables	13,669			11,634
Other current assets	625			602
Total current assets	28,806			25,853
Goodwill	8,071			8,171
Intangible assets, net	874			1,149
Fixed assets, net	606			588
Operating lease right-of-use assets	804
Deferred tax assets	150			144
Prepaid pension	1,208			1,128
Non-current intercompany receivables	7,198			7,225
Other non-current assets	525			417
Investment in subsidiary	(725)			(882)
Total assets	47,517			43,793
Current liabilities
Accounts payable and accrued liabilities	1,705			2,380
Short-term debt and current portion of long-term debt	3			1
Fiduciary liabilities	11,041			10,166
Current intercompany payables	1,688			634
Other current liabilities	1,084			867
Total current liabilities	15,521			14,048
Long-term debt	0			0
Non-current operating lease liabilities	776
Deferred tax liabilities	353			325
Pension, other postretirement, and postemployment liabilities	329			361
Non-current intercompany payables	489			499
Other non-current liabilities	843			927
Total liabilities	18,311			16,160
Total Aon shareholders’ equity	29,137			27,565
Noncontrolling interests	69			68
Total equity	29,206			27,633
Total liabilities and equity	47,517			43,793
Consolidation Adjustments
Current assets
Cash and cash equivalents	(1,419)			(781)
Short-term investments	0			0
Receivables, net	0			0
Fiduciary assets	0			0
Current intercompany receivables	(17,912)			(12,722)
Other current assets	0			0
Total current assets	(19,331)			(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	(145)			(144)
Prepaid pension	0			0
Non-current intercompany receivables	(7,853)			(7,889)
Other non-current assets	0			0
Investment in subsidiary	(28,412)			(26,683)
Total assets	(55,741)			(48,219)
Current liabilities
Accounts payable and accrued liabilities	(1,419)			(781)
Short-term debt and current portion of long-term debt	0			0
Fiduciary liabilities	0			0
Current intercompany payables	(17,912)			(12,722)
Other current liabilities	0			0
Total current liabilities	(19,331)			(13,503)
Long-term debt	0			0
Non-current operating lease liabilities	0
Deferred tax liabilities	(145)			(144)
Pension, other postretirement, and postemployment liabilities	0			0
Non-current intercompany payables	(7,853)			(7,889)
Other non-current liabilities	0			0
Total liabilities	(27,329)			(21,536)
Total Aon shareholders’ equity	(28,412)			(26,683)
Noncontrolling interests	0			0
Total equity	(28,412)			(26,683)
Total liabilities and equity	$ (55,741)			$ (48,219)