Guarantee of Registered Securities - Condensed Consolidating Statement of Financial Position (Details) - USD ($) $ in Millions	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017	Jun. 30, 2017
Current assets
Cash and cash equivalents	$ 487		$ 756
Short-term investments	173		529
Receivables, net	2,992	$ 2,730	2,478
Fiduciary assets	10,476		9,625
Current intercompany receivables	0		0
Other current assets	732	587	289
Total current assets	14,860		13,677
Goodwill	8,291		8,358
Intangible assets, net	1,363		1,733
Fixed assets, net	575		564
Deferred tax assets	452	261	389
Non-current intercompany receivables	0		0
Prepaid pension	1,272		1,060
Other non-current assets	404	452	307
Investment in subsidiary	0		0
Total assets	27,217		26,088
Current liabilities
Accounts payable and accrued liabilities	1,447	1,969	1,961
Short-term debt and current portion of long-term debt	799		299
Fiduciary liabilities	10,476		9,625
Current intercompany payables	0		0
Other current liabilities	1,121	883	870
Total current liabilities	13,843		12,755
Long-term debt	5,659		5,667
Deferred tax liabilities	294	169	127
Pension, other postretirement, and postemployment liabilities	1,715		1,789
Non-current intercompany payables	0		0
Other non-current liabilities	1,088	1,099	1,102
Total liabilities	22,599		21,440
Total Aon shareholders’ equity	4,545		4,583
Noncontrolling interests	73		65
Total equity	4,618	$ 5,155	4,648	$ 5,553
Total liabilities and equity	27,217		26,088
Aon plc
Current assets
Cash and cash equivalents	0		1
Short-term investments	0		0
Receivables, net	0		0
Fiduciary assets	0		0
Current intercompany receivables	171		165
Other current assets	0		1
Total current assets	171		167
Goodwill	0		0
Intangible assets, net	0		0
Fixed assets, net	0		0
Deferred tax assets	99		99
Non-current intercompany receivables	409		414
Prepaid pension	0		0
Other non-current assets	1		1
Investment in subsidiary	10,235		8,884
Total assets	10,915		9,565
Current liabilities
Accounts payable and accrued liabilities	1,452		574
Short-term debt and current portion of long-term debt	459		0
Fiduciary liabilities	0		0
Current intercompany payables	214		130
Other current liabilities	0		16
Total current liabilities	2,125		720
Long-term debt	4,243		4,251
Deferred tax liabilities	0		0
Pension, other postretirement, and postemployment liabilities	0		0
Non-current intercompany payables	0		0
Other non-current liabilities	2		11
Total liabilities	6,370		4,982
Total Aon shareholders’ equity	4,545		4,583
Noncontrolling interests	0		0
Total equity	4,545		4,583
Total liabilities and equity	10,915		9,565
Aon Corporation
Current assets
Cash and cash equivalents	2,927		2,524
Short-term investments	59		355
Receivables, net	0		2
Fiduciary assets	0		0
Current intercompany receivables	3,343		1,046
Other current assets	3		29
Total current assets	6,332		3,956
Goodwill	0		0
Intangible assets, net	0		0
Fixed assets, net	0		0
Deferred tax assets	410		396
Non-current intercompany receivables	261		261
Prepaid pension	6		6
Other non-current assets	28		35
Investment in subsidiary	19,008		17,909
Total assets	26,045		22,563
Current liabilities
Accounts payable and accrued liabilities	67		36
Short-term debt and current portion of long-term debt	339		0
Fiduciary liabilities	0		0
Current intercompany payables	13,364		11,149
Other current liabilities	62		64
Total current liabilities	13,832		11,249
Long-term debt	1,415		1,415
Deferred tax liabilities	0		0
Pension, other postretirement, and postemployment liabilities	1,349		1,391
Non-current intercompany payables	8,419		8,398
Other non-current liabilities	104		91
Total liabilities	25,119		22,544
Total Aon shareholders’ equity	926		19
Noncontrolling interests	0		0
Total equity	926		19
Total liabilities and equity	26,045		22,563
Other Non-Guarantor Subsidiaries
Current assets
Cash and cash equivalents	672		793
Short-term investments	114		174
Receivables, net	2,992		2,476
Fiduciary assets	10,476		9,625
Current intercompany receivables	11,424		10,824
Other current assets	729		259
Total current assets	26,407		24,151
Goodwill	8,291		8,358
Intangible assets, net	1,363		1,733
Fixed assets, net	575		564
Deferred tax assets	135		143
Non-current intercompany receivables	8,254		8,232
Prepaid pension	1,266		1,054
Other non-current assets	375		271
Investment in subsidiary	926		19
Total assets	47,592		44,525
Current liabilities
Accounts payable and accrued liabilities	3,040		3,913
Short-term debt and current portion of long-term debt	1		299
Fiduciary liabilities	10,476		9,625
Current intercompany payables	1,360		756
Other current liabilities	1,059		790
Total current liabilities	15,936		15,383
Long-term debt	1		1
Deferred tax liabilities	486		376
Pension, other postretirement, and postemployment liabilities	366		398
Non-current intercompany payables	505		509
Other non-current liabilities	982		1,000
Total liabilities	18,276		17,667
Total Aon shareholders’ equity	29,243		26,793
Noncontrolling interests	73		65
Total equity	29,316		26,858
Total liabilities and equity	47,592		44,525
Consolidation Adjustments
Current assets
Cash and cash equivalents	(3,112)		(2,562)
Short-term investments	0		0
Receivables, net	0		0
Fiduciary assets	0		0
Current intercompany receivables	(14,938)		(12,035)
Other current assets	0		0
Total current assets	(18,050)		(14,597)
Goodwill	0		0
Intangible assets, net	0		0
Fixed assets, net	0		0
Deferred tax assets	(192)		(249)
Non-current intercompany receivables	(8,924)		(8,907)
Prepaid pension	0		0
Other non-current assets	0		0
Investment in subsidiary	(30,169)		(26,812)
Total assets	(57,335)		(50,565)
Current liabilities
Accounts payable and accrued liabilities	(3,112)		(2,562)
Short-term debt and current portion of long-term debt	0		0
Fiduciary liabilities	0		0
Current intercompany payables	(14,938)		(12,035)
Other current liabilities	0		0
Total current liabilities	(18,050)		(14,597)
Long-term debt	0		0
Deferred tax liabilities	(192)		(249)
Pension, other postretirement, and postemployment liabilities	0		0
Non-current intercompany payables	(8,924)		(8,907)
Other non-current liabilities	0		0
Total liabilities	(27,166)		(23,753)
Total Aon shareholders’ equity	(30,169)		(26,812)
Noncontrolling interests	0		0
Total equity	(30,169)		(26,812)
Total liabilities and equity	$ (57,335)		$ (50,565)