Shareholders' Equity Components of Accumulated other comprehensive loss, net of related tax (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ (2,610)		$ (2,370)		$ (1,917)
Other comprehensive loss before reclassifications	286		(508)		(773)
Tax benefit	(143)		171		243
Other comprehensive loss before reclassifications, net	143		(337)		(530)
Amounts reclassified from accumulated other comprehensive loss	132		143		114
Tax benefit	(39)		(46)		(37)
Amounts reclassified from accumulated other comprehensive loss, net	93		97		77
Total other comprehensive gain (loss) attributable to Aon shareholders	236		(240)		(453)
Ending balance	(2,374)		(2,610)		(2,370)
Change in Fair Value of Investments [Member]
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	0	[1]	0	[1]	0	[1]
Other comprehensive loss before reclassifications	27	[1]	0	[1]	0	[1]
Tax benefit	(13)	[1]	0	[1]	0	[1]
Other comprehensive loss before reclassifications, net	14	[1]	0	[1]	0	[1]
Amounts reclassified from accumulated other comprehensive loss	(20)	[1]	0	[1]	0	[1]
Tax benefit	7	[1]	0	[1]	0	[1]
Amounts reclassified from accumulated other comprehensive loss, net	(13)	[1]	0	[1]	0	[1]
Total other comprehensive gain (loss) attributable to Aon shareholders	1	[1]	0	[1]	0	[1]
Ending balance	1	[1]	0	[1]	0	[1]
Change in Fair Value of Derivatives [Member]
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(28)	[1]	(37)	[1]	(24)	[1]
Other comprehensive loss before reclassifications	(12)	[1]	(19)	[1]	(44)	[1]
Tax benefit	5	[1]	7	[1]	15	[1]
Other comprehensive loss before reclassifications, net	(7)	[1]	(12)	[1]	(29)	[1]
Amounts reclassified from accumulated other comprehensive loss	21	[1]	33	[1]	25	[1]
Tax benefit	(8)	[1]	(12)	[1]	(9)	[1]
Amounts reclassified from accumulated other comprehensive loss, net	13	[1]	21	[1]	16	[1]
Total other comprehensive gain (loss) attributable to Aon shareholders	6	[1]	9	[1]	(13)	[1]
Ending balance	(22)	[1]	(28)	[1]	(37)	[1]
Foreign Currency Translation Adjustments [Member]
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	233		124		168
Other comprehensive loss before reclassifications	(65)		109		(47)
Tax benefit	1		0		3
Other comprehensive loss before reclassifications, net	(64)		109		(44)
Amounts reclassified from accumulated other comprehensive loss	0		0		0
Tax benefit	0		0		0
Amounts reclassified from accumulated other comprehensive loss, net	0		0		0
Total other comprehensive gain (loss) attributable to Aon shareholders	(64)		109		(44)
Ending balance	169		233		124
Post-retirement Benefit Obligations [Member]
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(2,815)	[2]	(2,457)	[2]	(2,061)	[2]
Other comprehensive loss before reclassifications	336	[2]	(598)	[2]	(682)
Tax benefit	(136)	[2]	164	[2]	225
Other comprehensive loss before reclassifications, net	200	[2]	(434)	[2]	(457)	[2]
Amounts reclassified from accumulated other comprehensive loss	131	[2]	110	[2]	89	[2]
Tax benefit	(38)	[2]	(34)	[2]	(28)	[2]
Amounts reclassified from accumulated other comprehensive loss, net	93	[2]	76	[2]	61	[2]
Total other comprehensive gain (loss) attributable to Aon shareholders	293	[2]	(358)	[2]	(396)	[2]
Ending balance	$ (2,522)	[2]	$ (2,815)	[2]	$ (2,457)	[2]

[1] Reclassifications from this category included in Accumulated other comprehensive loss are recorded in Other income

[2] Reclassifications from this category included in Accumulated other comprehensive loss are recorded in Compensation and benefits