Shareholders' Equity (Details 2) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Other comprehensive income (loss), pretax
Net derivative losses arising during the year, pretax	$ (19)	$ (44)	$ (31)
Reclassification adjustment, pretax	33	25	(5)
Net change in derivative losses, pretax	14	(19)	(36)
Net foreign exchange translation adjustments, pretax	109	(46)	(92)
Net post-retirement benefit obligation, pretax	(488)	(593)	(76)
Total other comprehensive loss, pretax	(365)	(658)	(204)
Less: other comprehensive income attributable to noncontrolling interest, pretax		1	(2)
Other comprehensive loss attributable to Aon stockholders, pretax	(365)	(659)	(202)
Other comprehensive income (loss), income tax benefit (expense)
Net derivative losses arising during the year, income tax benefit (expense)	7	15	10
Reclassification adjustment, income tax benefit (expense)	(12)	(9)	2
Net change in derivative losses, income tax benefit (expense)	(5)	6	12
Net foreign exchange translation adjustments, income tax benefit (expense)		3	(43)
Net post-retirement benefit obligation, income tax benefit (expense)	130	197	35
Total other comprehensive loss, income tax benefit (expense)	125	206	4
Other comprehensive loss attributable to Aon stockholders, income tax benefit (expense)	125	206	4
Other comprehensive gain (loss), net of tax:
Net derivative losses arising during the year, net of tax	(12)	(29)	(21)
Reclassification adjustment, net of tax	21	16	(3)
Net change in derivative gains/losses, net of tax	9	(13)	(24)
Net foreign exchange translation adjustments, net of tax	109	(43)	(135)
Net post-retirement benefit obligation	(358)	(396)	(41)
Total other comprehensive loss	(240)	(452)	(200)
Less: other comprehensive income attributable to noncontrolling interests, net of tax		1	(2)
Total other comprehensive loss attributable to Aon shareholders	(240)	(453)	(198)
Accumulated other comprehensive loss
Net derivative losses	(28)	(37)	(24)
Net foreign exchange translation adjustments	233	124	168
Net postretirement benefit obligations	(2,815)	(2,457)	(2,061)
Accumulated other comprehensive loss, net of tax	$ (2,610)	$ (2,370)	$ (1,917)