Guarantee of Registered Securities - Condensed Consolidating Statement of Financial Position (Details) - USD ($) $ in Millions	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017	Sep. 30, 2017
Current assets
Cash and cash equivalents	$ 484		$ 756
Short-term investments	167		529
Receivables, net	2,656	$ 2,730	2,478
Fiduciary assets	9,314		9,625
Current intercompany receivables	0		0
Other current assets	727	587	289
Total current assets	13,348		13,677
Goodwill	8,282		8,358
Intangible assets, net	1,260		1,733
Fixed assets, net	594		564
Deferred tax assets	476	261	389
Non-current intercompany receivables	0		0
Prepaid pension	1,208		1,060
Other non-current assets	434	452	307
Investment in subsidiary	0		0
Total assets	25,602		26,088
Current liabilities
Accounts payable and accrued liabilities	1,600	1,969	1,961
Short-term debt and current portion of long-term debt	741		299
Fiduciary liabilities	9,314		9,625
Current intercompany payables	0		0
Other current liabilities	988	883	870
Total current liabilities	12,643		12,755
Long-term debt	5,665		5,667
Deferred tax liabilities	273	169	127
Pension, other postretirement, and postemployment liabilities	1,603		1,789
Non-current intercompany payables	0		0
Other non-current liabilities	1,090	1,099	1,102
Total liabilities	21,274		21,440
Total Aon shareholders’ equity	4,262		4,583
Noncontrolling interests	66		65
Total equity	4,328	$ 5,155	4,648	$ 5,247
Total liabilities and equity	25,602		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	165		165
Other current assets	0		1
Total current assets	165		167
Goodwill	0		0
Intangible assets, net	0		0
Fixed assets, net	0		0
Deferred tax assets	99		99
Non-current intercompany receivables	412		414
Prepaid pension	0		0
Other non-current assets	1		1
Investment in subsidiary	10,086		8,884
Total assets	10,763		9,565
Current liabilities
Accounts payable and accrued liabilities	1,516		574
Short-term debt and current portion of long-term debt	540		0
Fiduciary liabilities	0		0
Current intercompany payables	193		130
Other current liabilities	0		16
Total current liabilities	2,249		720
Long-term debt	4,249		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	3		11
Total liabilities	6,501		4,982
Total Aon shareholders’ equity	4,262		4,583
Noncontrolling interests	0		0
Total equity	4,262		4,583
Total liabilities and equity	10,763		9,565
Aon Corporation
Current assets
Cash and cash equivalents	1,927		2,524
Short-term investments	47		355
Receivables, net	0		2
Fiduciary assets	0		0
Current intercompany receivables	4,175		1,046
Other current assets	15		29
Total current assets	6,164		3,956
Goodwill	0		0
Intangible assets, net	0		0
Fixed assets, net	0		0
Deferred tax assets	419		396
Non-current intercompany receivables	260		261
Prepaid pension	6		6
Other non-current assets	37		35
Investment in subsidiary	18,858		17,909
Total assets	25,744		22,563
Current liabilities
Accounts payable and accrued liabilities	66		36
Short-term debt and current portion of long-term debt	200		0
Fiduciary liabilities	0		0
Current intercompany payables	14,605		11,149
Other current liabilities	64		64
Total current liabilities	14,935		11,249
Long-term debt	1,416		1,415
Deferred tax liabilities	0		0
Pension, other postretirement, and postemployment liabilities	1,238		1,391
Non-current intercompany payables	8,421		8,398
Other non-current liabilities	105		91
Total liabilities	26,115		22,544
Total Aon shareholders’ equity	(371)		19
Noncontrolling interests	0		0
Total equity	(371)		19
Total liabilities and equity	25,744		22,563
Other Non-Guarantor Subsidiaries
Current assets
Cash and cash equivalents	400		793
Short-term investments	120		174
Receivables, net	2,656		2,476
Fiduciary assets	9,314		9,625
Current intercompany receivables	12,173		10,824
Other current assets	712		259
Total current assets	25,375		24,151
Goodwill	8,282		8,358
Intangible assets, net	1,260		1,733
Fixed assets, net	594		564
Deferred tax assets	153		143
Non-current intercompany receivables	8,256		8,232
Prepaid pension	1,202		1,054
Other non-current assets	396		271
Investment in subsidiary	(371)		19
Total assets	45,147		44,525
Current liabilities
Accounts payable and accrued liabilities	1,861		3,913
Short-term debt and current portion of long-term debt	1		299
Fiduciary liabilities	9,314		9,625
Current intercompany payables	1,715		756
Other current liabilities	924		790
Total current liabilities	13,815		15,383
Long-term debt	0		1
Deferred tax liabilities	468		376
Pension, other postretirement, and postemployment liabilities	365		398
Non-current intercompany payables	507		509
Other non-current liabilities	982		1,000
Total liabilities	16,137		17,667
Total Aon shareholders’ equity	28,944		26,793
Noncontrolling interests	66		65
Total equity	29,010		26,858
Total liabilities and equity	45,147		44,525
Consolidation Adjustments
Current assets
Cash and cash equivalents	(1,843)		(2,562)
Short-term investments	0		0
Receivables, net	0		0
Fiduciary assets	0		0
Current intercompany receivables	(16,513)		(12,035)
Other current assets	0		0
Total current assets	(18,356)		(14,597)
Goodwill	0		0
Intangible assets, net	0		0
Fixed assets, net	0		0
Deferred tax assets	(195)		(249)
Non-current intercompany receivables	(8,928)		(8,907)
Prepaid pension	0		0
Other non-current assets	0		0
Investment in subsidiary	(28,573)		(26,812)
Total assets	(56,052)		(50,565)
Current liabilities
Accounts payable and accrued liabilities	(1,843)		(2,562)
Short-term debt and current portion of long-term debt	0		0
Fiduciary liabilities	0		0
Current intercompany payables	(16,513)		(12,035)
Other current liabilities	0		0
Total current liabilities	(18,356)		(14,597)
Long-term debt	0		0
Deferred tax liabilities	(195)		(249)
Pension, other postretirement, and postemployment liabilities	0		0
Non-current intercompany payables	(8,928)		(8,907)
Other non-current liabilities	0		0
Total liabilities	(27,479)		(23,753)
Total Aon shareholders’ equity	(28,573)		(26,812)
Noncontrolling interests	0		0
Total equity	(28,573)		(26,812)
Total liabilities and equity	$ (56,052)		$ (50,565)