Guarantee of Registered Securities (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Aon plc	Dec. 31, 2011 Aon plc	Dec. 31, 2010 Aon plc	Dec. 31, 2012 Aon Corporation	Dec. 31, 2011 Aon Corporation	Dec. 31, 2010 Aon Corporation	Dec. 31, 2012 Other Non-Guarantor Subsidiaries	Dec. 31, 2011 Other Non-Guarantor Subsidiaries	Dec. 31, 2010 Other Non-Guarantor Subsidiaries	Dec. 31, 2012 Consolidating Adjustments	Dec. 31, 2011 Consolidating Adjustments	Dec. 31, 2010 Consolidating Adjustments	Dec. 31, 2012 7.375% Debt securities due December 2012	Dec. 31, 2012 3.50% senior notes due September 2015	Sep. 07, 2010 3.50% senior notes due September 2015	Dec. 31, 2012 3.125% Senior notes due May 2016	May 24, 2011 3.125% Senior notes due May 2016	Dec. 31, 2012 5.00% Senior notes due September 2020	Sep. 07, 2010 5.00% Senior notes due September 2020	Dec. 31, 2012 8.205% Junior subordinated deferrable interest debentures due January 2027	Dec. 31, 2012 6.25% Senior notes due September 2040	Sep. 07, 2010 6.25% Senior notes due September 2040	Dec. 31, 2012 4.250% senior notes due 2042	Dec. 12, 2012 4.250% senior notes due 2042
Financial Information for Subsidiary Guarantors and Non Guarantors
Parent company's percentage ownership of guarantors												100.00%
Debt interest rate (as a percent)																								7.375%	3.50%	3.50%	3.125%	3.125%	5.00%	5.00%	8.205%	6.25%	6.25%	4.25%	4.25%
Revenue
Commissions, fees and other	$ 3,108	$ 2,726	$ 2,813	$ 2,829	$ 2,980	$ 2,708	$ 2,799	$ 2,748	$ 11,476	$ 11,235	$ 8,457	$ 2			$ 1			$ 11,474	$ 11,236	$ 8,459	$ (1)	$ (1)	$ (2)
Fiduciary investment income	7	11	8	12	14	15	12	11	38	52	55				1	2	15	37	50	40
Total revenue	3,115	2,737	2,821	2,841	2,994	2,723	2,811	2,759	11,514	11,287	8,512	2			2	2	15	11,511	11,286	8,499	(1)	(1)	(2)
Expenses
Compensation and benefits									6,709	6,567	5,097	117			24	118	47	6,568	6,449	5,050
Other general expenses									3,209	3,124	2,171	18			47	47	50	3,145	3,078	2,123	(1)	(1)	(2)
Total operating expenses									9,918	9,691	7,268	135			71	165	97	9,713	9,527	7,173	(1)	(1)	(2)
Operating income	461	339	394	402	430	328	440	398	1,596	1,596	1,244	(133)			(69)	(163)	(82)	1,798	1,759	1,326
Interest income									10	18	15				1	4	7	9	14	8
Interest expense									(228)	(245)	(182)				(164)	(171)	(105)	(64)	(74)	(77)
Intercompany interest (expense) income												(19)			190	180	133	(171)	(180)	(133)
Other (expense) income									3	15	(18)				57	32	14	(54)	(17)	(32)
(Loss) income from continuing operations before income taxes									1,381	1,384	1,059	(152)			15	(118)	(33)	1,518	1,502	1,092
Income tax (benefit) expense									360	378	300	(37)			8	(43)	(14)	389	421	314
Income from continuing operations	307	210	255	249	280	208	265	253	1,021	1,006	759	(115)			7	(75)	(19)	1,129	1,081	778
Income (Loss) from discontinued operations before taxes									(1)	5	(39)					5	(39)	(1)
Income tax expense										1	(12)					1	(12)
(Loss) income from discontinued operations			(1)				2	2	(1)	4	(27)					4	(27)	(1)
Equity in earnings of subsidiaries, net of tax												1,108	979	706	988	921	622	995	850	576	(3,091)	(2,750)	(1,904)
Net income	307	210	254	249	280	208	267	255	1,020	1,010	732	993	979	706	995	850	576	2,123	1,931	1,354	(3,091)	(2,750)	(1,904)
Less: Net income attributable to noncontrolling interests	2	6	8	11	3	10	9	9	27	31	26							27	31	26
Net income attributable to Aon shareholders	305	204	246	238	277	198	258	246	993	979	706	993	979	706	995	850	576	2,096	1,900	1,328	(3,091)	(2,750)	(1,904)
Comprehensive income attributable to Aon stockholders
Net income	307	210	254	249	280	208	267	255	1,020	1,010	732	993	979	706	995	850	576	2,123	1,931	1,354	(3,091)	(2,750)	(1,904)
Less: Net income attributable to noncontrolling interests	2	6	8	11	3	10	9	9	27	31	26							27	31	26
Net income attributable to Aon stockholders	305	204	246	238	277	198	258	246	993	979	706	993	979	706	995	850	576	2,096	1,900	1,328	(3,091)	(2,750)	(1,904)
Other comprehensive income, net of tax:
Change in accounting principle											(44)						(44)
Change in fair value of derivatives									9	(13)	(24)				2		(17)	7	(13)	(7)
Foreign currency translation adjustments									109	(43)	(135)				24	(6)	125	85	(37)	(260)
Post-retirement benefit obligation									(358)	(396)	(41)				(67)	(173)	(62)	(291)	(223)	21
Total other comprehensive loss									(240)	(452)	(244)				(41)	(179)	2	(199)	(273)	(246)
Equity in other comprehensive (loss) income of subsidiaries, net of tax												(240)	(453)	(242)	(200)	(277)	(231)	(241)	(456)	(229)	681	1,186	702
Less: Other comprehensive income (loss) attributable to noncontrolling interests										1	(2)								1	(2)
Total other comprehensive loss attributable to Aon shareholders									(240)	(453)	(242)	(240)	(453)	(242)	(241)	(456)	(229)	(440)	(730)	(473)	681	1,186	702
Comprehensive income attributable to Aon shareholders									$ 753	$ 526	$ 464	$ 753	$ 526	$ 464	$ 754	$ 394	$ 347	$ 1,656	$ 1,170	$ 855	$ (2,410)	$ (1,564)	$ (1,202)