Shareholders' Equity - Components of Accumulated other comprehensive loss, net of related tax (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ (2,374)		$ (2,610)		$ (2,370)
Other comprehensive loss before reclassifications	(1,068)		286		(508)
Tax benefit	221		(143)		171
Other comprehensive loss before reclassifications, net	(847)		143		(337)
Amounts reclassified from accumulated other comprehensive loss	126		132		143
Tax benefit	(39)		(39)		(46)
Amounts reclassified from accumulated other comprehensive loss, net	87		93		97
Total other comprehensive (loss) income attributable to Aon shareholders	(760)		236		(240)
Ending balance	(3,134)		(2,374)		(2,610)
Change in Fair Value of Investments [Member]
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	1	[1]	0	[1]	0	[1]
Other comprehensive loss before reclassifications	(2)	[1]	27	[1]	0	[1]
Tax benefit	1	[1]	(13)	[1]	0	[1]
Other comprehensive loss before reclassifications, net	(1)	[1]	14	[1]	0	[1]
Amounts reclassified from accumulated other comprehensive loss	0	[1]	(20)	[1]	0	[1]
Tax benefit	0	[1]	7	[1]	0	[1]
Amounts reclassified from accumulated other comprehensive loss, net	0	[1]	(13)	[1]	0	[1]
Total other comprehensive (loss) income attributable to Aon shareholders	(1)	[1]	1	[1]	0	[1]
Ending balance	0	[1]	1	[1]	0	[1]
Change in Fair Value of Derivatives [Member]
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(22)	[1]	(28)	[1]	(37)	[1]
Other comprehensive loss before reclassifications	(11)	[1]	(12)	[1]	(19)	[1]
Tax benefit	3	[1]	5	[1]	7	[1]
Other comprehensive loss before reclassifications, net	(8)	[1]	(7)	[1]	(12)	[1]
Amounts reclassified from accumulated other comprehensive loss	20	[1]	21	[1]	33	[1]
Tax benefit	(7)	[1]	(8)	[1]	(12)	[1]
Amounts reclassified from accumulated other comprehensive loss, net	13	[1]	13	[1]	21	[1]
Total other comprehensive (loss) income attributable to Aon shareholders	5	[1]	6	[1]	9	[1]
Ending balance	(17)	[1]	(22)	[1]	(28)	[1]
Foreign Currency Translation Adjustments [Member]
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	169		233		124
Other comprehensive loss before reclassifications	(492)		(65)		109
Tax benefit	(12)		1		0
Other comprehensive loss before reclassifications, net	(504)		(64)		109
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 (loss) income attributable to Aon shareholders	(504)		(64)		109
Ending balance	(335)		169		233
Post-retirement Benefit Obligations [Member]
Changes in Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(2,522)	[2]	(2,815)	[2]	(2,457)	[2]
Other comprehensive loss before reclassifications	(563)	[2]	336	[2]	(598)	[2]
Tax benefit	229	[2]	(136)	[2]	164	[2]
Other comprehensive loss before reclassifications, net	(334)	[2]	200	[2]	(434)	[2]
Amounts reclassified from accumulated other comprehensive loss	106	[2]	131	[2]	110	[2]
Tax benefit	(32)	[2]	(38)	[2]	(34)	[2]
Amounts reclassified from accumulated other comprehensive loss, net	74	[2]	93	[2]	76	[2]
Total other comprehensive (loss) income attributable to Aon shareholders	(260)	[2]	293	[2]	(358)	[2]
Ending balance	$ (2,782)	[2]	$ (2,522)	[2]	$ (2,815)	[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