Guarantee of Registered Securities - Condensed Consolidating Statement of Financial Position (Details) - USD ($) $ in Millions	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
CURRENT ASSETS
Cash and cash equivalents	$ 597		$ 756
Short-term investments	118		529
Receivables, net	3,053	$ 2,730	2,478
Fiduciary assets	10,738		9,625
Current intercompany receivables	0		0
Other current assets	609	587	289
Total Current Assets	15,115		13,677
Goodwill	8,550		8,358
Intangible assets, net	1,662		1,733
Fixed assets, net	578		564
Deferred tax assets	296	261	389
Non-current intercompany receivables	0		0
Prepaid pension	1,207		1,060
Other non-current assets	439	452	307
Investment in subsidiary	0		0
TOTAL ASSETS	27,847		26,088
CURRENT LIABILITIES
Accounts payable and accrued liabilities	1,545	1,969	1,961
Short-term debt and current portion of long-term debt	403		299
Fiduciary liabilities	10,738		9,625
Current intercompany payables	0		0
Other current liabilities	972	883	870
Total Current Liabilities	13,658		12,755
Long-term debt	5,697		5,667
Deferred tax liabilities	243	169	127
Pension, other postretirement, and postemployment liabilities	1,759		1,789
Non-current intercompany payables	0		0
Other non-current liabilities	1,105	1,099	1,102
TOTAL LIABILITIES	22,462		21,440
TOTAL AON SHAREHOLDERS’ EQUITY	5,301		4,583
Noncontrolling interests	84		65
TOTAL EQUITY	5,385	$ 5,155	4,648	$ 5,826	$ 5,532
TOTAL LIABILITIES AND EQUITY	27,847		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	186		165
Other current assets	1		1
Total Current Assets	187		167
Goodwill	0		0
Intangible assets, net	0		0
Fixed assets, net	0		0
Deferred tax assets	99		99
Non-current intercompany receivables	426		414
Prepaid pension	0		0
Other non-current assets	1		1
Investment in subsidiary	10,406		8,884
TOTAL ASSETS	11,119		9,565
CURRENT LIABILITIES
Accounts payable and accrued liabilities	896		574
Short-term debt and current portion of long-term debt	374		0
Fiduciary liabilities	0		0
Current intercompany payables	262		130
Other current liabilities	0		16
Total Current Liabilities	1,532		720
Long-term debt	4,282		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	4		11
TOTAL LIABILITIES	5,818		4,982
TOTAL AON SHAREHOLDERS’ EQUITY	5,301		4,583
Noncontrolling interests	0		0
TOTAL EQUITY	5,301		4,583
TOTAL LIABILITIES AND EQUITY	11,119		9,565
Aon Corporation
CURRENT ASSETS
Cash and cash equivalents	2,603		2,524
Short-term investments	1		355
Receivables, net	1		2
Fiduciary assets	0		0
Current intercompany receivables	2,355		1,046
Other current assets	19		29
Total Current Assets	4,979		3,956
Goodwill	0		0
Intangible assets, net	0		0
Fixed assets, net	0		0
Deferred tax assets	391		396
Non-current intercompany receivables	261		261
Prepaid pension	6		6
Other non-current assets	32		35
Investment in subsidiary	19,124		17,910
TOTAL ASSETS	24,793		22,564
CURRENT LIABILITIES
Accounts payable and accrued liabilities	12		36
Short-term debt and current portion of long-term debt	26		0
Fiduciary liabilities	0		0
Current intercompany payables	12,098		11,149
Other current liabilities	65		64
Total Current Liabilities	12,201		11,249
Long-term debt	1,414		1,415
Deferred tax liabilities	0		0
Pension, other postretirement, and postemployment liabilities	1,365		1,391
Non-current intercompany payables	8,412		8,398
Other non-current liabilities	90		91
TOTAL LIABILITIES	23,482		22,544
TOTAL AON SHAREHOLDERS’ EQUITY	1,311		20
Noncontrolling interests	0		0
TOTAL EQUITY	1,311		20
TOTAL LIABILITIES AND EQUITY	24,793		22,564
Other Non-Guarantor Subsidiaries
CURRENT ASSETS
Cash and cash equivalents	640		793
Short-term investments	117		174
Receivables, net	3,052		2,476
Fiduciary assets	10,738		9,625
Current intercompany receivables	10,953		10,824
Other current assets	589		259
Total Current Assets	26,089		24,151
Goodwill	8,550		8,358
Intangible assets, net	1,662		1,733
Fixed assets, net	578		564
Deferred tax assets	57		143
Non-current intercompany receivables	8,247		8,232
Prepaid pension	1,201		1,054
Other non-current assets	406		271
Investment in subsidiary	1,311		20
TOTAL ASSETS	48,101		44,526
CURRENT LIABILITIES
Accounts payable and accrued liabilities	3,283		3,913
Short-term debt and current portion of long-term debt	3		299
Fiduciary liabilities	10,738		9,625
Current intercompany payables	1,134		756
Other current liabilities	907		790
Total Current Liabilities	16,065		15,383
Long-term debt	1		1
Deferred tax liabilities	494		376
Pension, other postretirement, and postemployment liabilities	394		398
Non-current intercompany payables	522		509
Other non-current liabilities	1,011		1,000
TOTAL LIABILITIES	18,487		17,667
TOTAL AON SHAREHOLDERS’ EQUITY	29,530		26,794
Noncontrolling interests	84		65
TOTAL EQUITY	29,614		26,859
TOTAL LIABILITIES AND EQUITY	48,101		44,526
Consolidation Adjustments
CURRENT ASSETS
Cash and cash equivalents	(2,646)		(2,562)
Short-term investments	0		0
Receivables, net	0		0
Fiduciary assets	0		0
Current intercompany receivables	(13,494)		(12,035)
Other current assets	0		0
Total Current Assets	(16,140)		(14,597)
Goodwill	0		0
Intangible assets, net	0		0
Fixed assets, net	0		0
Deferred tax assets	(251)		(249)
Non-current intercompany receivables	(8,934)		(8,907)
Prepaid pension	0		0
Other non-current assets	0		0
Investment in subsidiary	(30,841)		(26,814)
TOTAL ASSETS	(56,166)		(50,567)
CURRENT LIABILITIES
Accounts payable and accrued liabilities	(2,646)		(2,562)
Short-term debt and current portion of long-term debt	0		0
Fiduciary liabilities	0		0
Current intercompany payables	(13,494)		(12,035)
Other current liabilities	0		0
Total Current Liabilities	(16,140)		(14,597)
Long-term debt	0		0
Deferred tax liabilities	(251)		(249)
Pension, other postretirement, and postemployment liabilities	0		0
Non-current intercompany payables	(8,934)		(8,907)
Other non-current liabilities	0		0
TOTAL LIABILITIES	(25,325)		(23,753)
TOTAL AON SHAREHOLDERS’ EQUITY	(30,841)		(26,814)
Noncontrolling interests	0		0
TOTAL EQUITY	(30,841)		(26,814)
TOTAL LIABILITIES AND EQUITY	$ (56,166)		$ (50,567)