Guarantee of Registered Securities - Condensed Consolidating Statement of Financial Position (Details) - USD ($) $ in Millions	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Total current assets
Cash and cash equivalents	$ 656		$ 756
Short-term investments	172		529
Receivables, net	2,760	$ 2,730	2,478
Fiduciary assets	10,166		9,625
Intercompany receivables	0		0
Other current assets	618	587	289
Total current assets	14,372		13,677
Goodwill	8,171		8,358	$ 7,410
Intangible assets, net	1,149		1,733
Fixed assets, net	588		564
Deferred tax assets	561	261	389
Deferred tax assets	0		0
Prepaid pension	1,133		1,060
Other non-current assets	448	452	307
Investment in subsidiary	0		0
Total assets	26,422		26,088
Current liabilities
Accounts payable and accrued liabilities	1,943		1,961
Short-term debt and current portion of long-term debt	251		299
Fiduciary liabilities	10,166	1,969	9,625
Intercompany payables	0		0
Other current liabilities	936	883	870
Total current liabilities	13,296		12,755
Long-term debt	5,993		5,667
Deferred tax liabilities	181	169	127
Pension, other postretirement and other post-employment liabilities	1,636		1,789
Intercompany payables	0		0
Other non-current liabilities	1,097	1,099	1,102
Total liabilities	22,203		21,440
Total Aon shareholders' equity	4,151		4,583
Noncontrolling interests	68		65
Total equity	4,219	$ 5,155	4,648	$ 5,532	$ 6,059
Total liabilities and equity	26,422		26,088
Aon plc
Total current assets
Cash and cash equivalents	0		1
Short-term investments	0		0
Receivables, net	0		0
Fiduciary assets	0		0
Intercompany receivables	191		165
Other current assets	0		1
Total current assets	191		167
Goodwill	0		0
Intangible assets, net	0		0
Fixed assets, net	0		0
Deferred tax assets	94		99
Deferred tax assets	403		414
Prepaid pension	0		0
Other non-current assets	1		1
Investment in subsidiary	8,433		8,884
Total assets	9,122		9,565
Current liabilities
Accounts payable and accrued liabilities	274		574
Short-term debt and current portion of long-term debt	250		0
Fiduciary liabilities	0		0
Intercompany payables	213		130
Other current liabilities	0		16
Total current liabilities	737		720
Long-term debt	4,231		4,251
Deferred tax liabilities	0		0
Pension, other postretirement and other post-employment liabilities	0		0
Intercompany payables	0		0
Other non-current liabilities	3		11
Total liabilities	4,971		4,982
Total Aon shareholders' equity	4,151		4,583
Noncontrolling interests	0		0
Total equity	4,151		4,583
Total liabilities and equity	9,122		9,565
Aon Corporation
Total current assets
Cash and cash equivalents	862		2,524
Short-term investments	56		355
Receivables, net	0		2
Fiduciary assets	0		0
Intercompany receivables	897		1,046
Other current assets	16		29
Total current assets	1,831		3,956
Goodwill	0		0
Intangible assets, net	0		0
Fixed assets, net	0		0
Deferred tax assets	467		396
Deferred tax assets	261		261
Prepaid pension	5		6
Other non-current assets	30		35
Investment in subsidiary	19,065		17,799
Total assets	21,659		22,453
Current liabilities
Accounts payable and accrued liabilities	70		36
Short-term debt and current portion of long-term debt	0		0
Fiduciary liabilities	0		0
Intercompany payables	11,695		11,149
Other current liabilities	69		64
Total current liabilities	11,834		11,249
Long-term debt	1,762		1,415
Deferred tax liabilities	0		0
Pension, other postretirement and other post-employment liabilities	1,275		1,391
Intercompany payables	7,570		8,398
Other non-current liabilities	115		91
Total liabilities	22,556		22,544
Total Aon shareholders' equity	(897)		(91)
Noncontrolling interests	0		0
Total equity	(897)		(91)
Total liabilities and equity	21,659		22,453
Other Non-Guarantor Subsidiaries
Total current assets
Cash and cash equivalents	575		793
Short-term investments	116		174
Receivables, net	2,760		2,476
Fiduciary assets	10,166		9,625
Intercompany receivables	11,454		10,824
Other current assets	602		259
Total current assets	25,673		24,151
Goodwill	8,171		8,358
Intangible assets, net	1,149		1,733
Fixed assets, net	588		564
Deferred tax assets	144		143
Deferred tax assets	7,405		8,232
Prepaid pension	1,128		1,054
Other non-current assets	417		271
Investment in subsidiary	(897)		(91)
Total assets	43,778		44,415
Current liabilities
Accounts payable and accrued liabilities	2,380		3,913
Short-term debt and current portion of long-term debt	1		299
Fiduciary liabilities	10,166		9,625
Intercompany payables	634		756
Other current liabilities	867		790
Total current liabilities	14,048		15,383
Long-term debt	0		1
Deferred tax liabilities	325		376
Pension, other postretirement and other post-employment liabilities	361		398
Intercompany payables	499		509
Other non-current liabilities	979		1,000
Total liabilities	16,212		17,667
Total Aon shareholders' equity	27,498		26,683
Noncontrolling interests	68		65
Total equity	27,566		26,748
Total liabilities and equity	43,778		44,415
Consolidating Adjustments
Total current assets
Cash and cash equivalents	(781)		(2,562)
Short-term investments	0		0
Receivables, net	0		0
Fiduciary assets	0		0
Intercompany receivables	(12,542)		(12,035)
Other current assets	0		0
Total current assets	(13,323)		(14,597)
Goodwill	0		0
Intangible assets, net	0		0
Fixed assets, net	0		0
Deferred tax assets	(144)		(249)
Deferred tax assets	(8,069)		(8,907)
Prepaid pension	0		0
Other non-current assets	0		0
Investment in subsidiary	(26,601)		(26,592)
Total assets	(48,137)		(50,345)
Current liabilities
Accounts payable and accrued liabilities	(781)		(2,562)
Short-term debt and current portion of long-term debt	0		0
Fiduciary liabilities	0		0
Intercompany payables	(12,542)		(12,035)
Other current liabilities	0		0
Total current liabilities	(13,323)		(14,597)
Long-term debt	0		0
Deferred tax liabilities	(144)		(249)
Pension, other postretirement and other post-employment liabilities	0		0
Intercompany payables	(8,069)		(8,907)
Other non-current liabilities	0		0
Total liabilities	(21,536)		(23,753)
Total Aon shareholders' equity	(26,601)		(26,592)
Noncontrolling interests	0		0
Total equity	(26,601)		(26,592)
Total liabilities and equity	$ (48,137)		$ (50,345)