Guarantee of Registered Securities (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Revenues
Commissions, fees and other	$ 2,726,000,000	$ 2,708,000,000	$ 8,368,000,000	$ 8,255,000,000
Fiduciary investment income	11,000,000	15,000,000	31,000,000	38,000,000
Total revenue	2,737,000,000	2,723,000,000	8,399,000,000	8,293,000,000
Expenses
Compensation and benefits	1,620,000,000	1,634,000,000	4,920,000,000	4,843,000,000
Other general expenses	778,000,000	761,000,000	2,344,000,000	2,284,000,000
Total operating expenses	2,398,000,000	2,395,000,000	7,264,000,000	7,127,000,000
Operating income	339,000,000	328,000,000	1,135,000,000	1,166,000,000
Interest income	1,000,000	4,000,000	6,000,000	14,000,000
Interest expense	(57,000,000)	(60,000,000)	(173,000,000)	(186,000,000)
Other (expense) income	(9,000,000)	20,000,000	3,000,000	6,000,000
Income from continuing operations before income taxes	274,000,000	292,000,000	971,000,000	1,000,000,000
Income tax (benefit) expense	64,000,000	84,000,000	257,000,000	274,000,000
Income from continuing operations	210,000,000	208,000,000	714,000,000	726,000,000
Loss from discontinued operations before income taxes			(1,000,000)	5,000,000
Income taxes				1,000,000
(Loss) income from discontinued operations			(1,000,000)	4,000,000
Net income	210,000,000	208,000,000	713,000,000	730,000,000
Less: Net income attributable to noncontrolling interests	6,000,000	10,000,000	25,000,000	28,000,000
Net income attributable to Aon shareholders	204,000,000	198,000,000	688,000,000	702,000,000
Comprehensive income attributable to Aon stockholders
Net (loss) income	210,000,000	208,000,000	713,000,000	730,000,000
Less: Net income attributable to noncontrolling interests	6,000,000	10,000,000	25,000,000	28,000,000
Net income attributable to Aon stockholders	204,000,000	198,000,000	688,000,000	702,000,000
Other comprehensive income (loss), net of tax:
Change in fair value of derivatives	15,000,000	(9,000,000)	5,000,000	(14,000,000)
Foreign currency translation adjustments	164,000,000	(209,000,000)	71,000,000	(12,000,000)
Post-retirement benefit obligation	7,000,000	15,000,000	46,000,000	42,000,000
Total other comprehensive income (loss)	186,000,000	(203,000,000)	122,000,000	16,000,000
Less: Other comprehensive loss attributable to noncontrolling interests	4,000,000	(1,000,000)	2,000,000	(1,000,000)
Total other comprehensive income (loss) attributable to Aon shareholders	182,000,000	(202,000,000)	120,000,000	17,000,000
Comprehensive income (loss) attributable to Aon shareholders	386,000,000	(4,000,000)	808,000,000	719,000,000
Parent Guarantor
Financial Information for Subsidiary Guarantors and Non Guarantors
Parent company's percentage ownership of guarantors	100.00%		100.00%
Expenses
Compensation and benefits	34,000,000		18,000,000
Other general expenses	19,000,000		21,000,000
Total operating expenses	53,000,000		39,000,000
Operating income	(53,000,000)		(39,000,000)
Intercompany interest (expense) income	(8,000,000)		(10,000,000)
Income from continuing operations before income taxes	(61,000,000)		(49,000,000)
Income tax (benefit) expense	(15,000,000)
Income from continuing operations	(46,000,000)		(49,000,000)
Equity in earnings of subsidiaries, net of tax	250,000,000	198,000,000	737,000,000	702,000,000
Net income	204,000,000	198,000,000	688,000,000	702,000,000
Net income attributable to Aon shareholders	204,000,000	198,000,000	688,000,000	702,000,000
Comprehensive income attributable to Aon stockholders
Net (loss) income	204,000,000	198,000,000	688,000,000	702,000,000
Net income attributable to Aon stockholders	204,000,000	198,000,000	688,000,000	702,000,000
Other comprehensive income (loss), net of tax:
Equity in other comprehensive (loss) income of subsidiary, net of tax	182,000,000	(202,000,000)	120,000,000	17,000,000
Total other comprehensive income (loss) attributable to Aon shareholders	182,000,000	(202,000,000)	120,000,000	17,000,000
Comprehensive income (loss) attributable to Aon shareholders	386,000,000	(4,000,000)	808,000,000	719,000,000
Subsidiary Issuer
Revenues
Fiduciary investment income				2,000,000
Total revenue				2,000,000
Expenses
Compensation and benefits	(11,000,000)	(2,000,000)	22,000,000	(7,000,000)
Other general expenses	6,000,000	20,000,000	26,000,000	40,000,000
Total operating expenses	(5,000,000)	18,000,000	48,000,000	33,000,000
Operating income	5,000,000	(18,000,000)	(48,000,000)	(31,000,000)
Interest income			1,000,000	4,000,000
Interest expense	(44,000,000)	(40,000,000)	(124,000,000)	(131,000,000)
Intercompany interest (expense) income	48,000,000	47,000,000	140,000,000	135,000,000
Other (expense) income	10,000,000		12,000,000	(27,000,000)
Income from continuing operations before income taxes	19,000,000	(11,000,000)	(19,000,000)	(50,000,000)
Income tax (benefit) expense	11,000,000	(3,000,000)	(4,000,000)	(18,000,000)
Income from continuing operations	8,000,000	(8,000,000)	(15,000,000)	(32,000,000)
Loss from discontinued operations before income taxes				5,000,000
Income taxes				1,000,000
(Loss) income from discontinued operations				4,000,000
Equity in earnings of subsidiaries, net of tax	209,000,000	174,000,000	662,000,000	628,000,000
Net income	217,000,000	166,000,000	647,000,000	600,000,000
Net income attributable to Aon shareholders	217,000,000	166,000,000	647,000,000	600,000,000
Comprehensive income attributable to Aon stockholders
Net (loss) income	217,000,000	166,000,000	647,000,000	600,000,000
Net income attributable to Aon stockholders	217,000,000	166,000,000	647,000,000	600,000,000
Other comprehensive income (loss), net of tax:
Change in fair value of derivatives	1,000,000		1,000,000	(1,000,000)
Foreign currency translation adjustments	17,000,000	(5,000,000)	16,000,000	(9,000,000)
Post-retirement benefit obligation	9,000,000	4,000,000	24,000,000	13,000,000
Total other comprehensive income (loss)	27,000,000	(1,000,000)	41,000,000	3,000,000
Equity in other comprehensive (loss) income of subsidiary, net of tax	155,000,000	(201,000,000)	78,000,000	12,000,000
Total other comprehensive income (loss) attributable to Aon shareholders	182,000,000	(202,000,000)	119,000,000	15,000,000
Comprehensive income (loss) attributable to Aon shareholders	399,000,000	(36,000,000)	766,000,000	615,000,000
Other non-guarantor subsidiaries
Revenues
Commissions, fees and other	2,726,000,000	2,708,000,000	8,368,000,000	8,255,000,000
Fiduciary investment income	11,000,000	15,000,000	31,000,000	36,000,000
Total revenue	2,737,000,000	2,723,000,000	8,399,000,000	8,291,000,000
Expenses
Compensation and benefits	1,597,000,000	1,636,000,000	4,880,000,000	4,850,000,000
Other general expenses	753,000,000	741,000,000	2,297,000,000	2,245,000,000
Total operating expenses	2,350,000,000	2,377,000,000	7,177,000,000	7,095,000,000
Operating income	387,000,000	346,000,000	1,222,000,000	1,196,000,000
Interest income	1,000,000	4,000,000	5,000,000	10,000,000
Interest expense	(13,000,000)	(20,000,000)	(49,000,000)	(55,000,000)
Intercompany interest (expense) income	(40,000,000)	(47,000,000)	(130,000,000)	(135,000,000)
Other (expense) income	(19,000,000)	20,000,000	(9,000,000)	33,000,000
Income from continuing operations before income taxes	316,000,000	303,000,000	1,039,000,000	1,049,000,000
Income tax (benefit) expense	68,000,000	87,000,000	261,000,000	292,000,000
Income from continuing operations	248,000,000	216,000,000	778,000,000	757,000,000
Loss from discontinued operations before income taxes			(1,000,000)
(Loss) income from discontinued operations			(1,000,000)
Equity in earnings of subsidiaries, net of tax	217,000,000	166,000,000	647,000,000	600,000,000
Net income	465,000,000	382,000,000	1,424,000,000	1,357,000,000
Less: Net income attributable to noncontrolling interests	6,000,000	10,000,000	25,000,000	28,000,000
Net income attributable to Aon shareholders	459,000,000	372,000,000	1,399,000,000	1,329,000,000
Comprehensive income attributable to Aon stockholders
Net (loss) income	465,000,000	382,000,000	1,424,000,000	1,357,000,000
Less: Net income attributable to noncontrolling interests	6,000,000	10,000,000	25,000,000	28,000,000
Net income attributable to Aon stockholders	459,000,000	372,000,000	1,399,000,000	1,329,000,000
Other comprehensive income (loss), net of tax:
Change in fair value of derivatives	14,000,000	(9,000,000)	4,000,000	(13,000,000)
Foreign currency translation adjustments	147,000,000	(204,000,000)	55,000,000	(3,000,000)
Post-retirement benefit obligation	(2,000,000)	11,000,000	22,000,000	29,000,000
Total other comprehensive income (loss)	159,000,000	(202,000,000)	81,000,000	13,000,000
Equity in other comprehensive (loss) income of subsidiary, net of tax	182,000,000	(202,000,000)	119,000,000	15,000,000
Less: Other comprehensive loss attributable to noncontrolling interests	4,000,000	(1,000,000)	2,000,000	(1,000,000)
Total other comprehensive income (loss) attributable to Aon shareholders	337,000,000	(403,000,000)	198,000,000	29,000,000
Comprehensive income (loss) attributable to Aon shareholders	796,000,000	(31,000,000)	1,597,000,000	1,358,000,000
Consolidating adjustments
Expenses
Equity in earnings of subsidiaries, net of tax	(676,000,000)	(538,000,000)	(2,046,000,000)	(1,930,000,000)
Net income	(676,000,000)	(538,000,000)	(2,046,000,000)	(1,930,000,000)
Net income attributable to Aon shareholders	(676,000,000)	(538,000,000)	(2,046,000,000)	(1,930,000,000)
Comprehensive income attributable to Aon stockholders
Net (loss) income	(676,000,000)	(538,000,000)	(2,046,000,000)	(1,930,000,000)
Net income attributable to Aon stockholders	(676,000,000)	(538,000,000)	(2,046,000,000)	(1,930,000,000)
Other comprehensive income (loss), net of tax:
Equity in other comprehensive (loss) income of subsidiary, net of tax	(519,000,000)	605,000,000	(317,000,000)	(44,000,000)
Total other comprehensive income (loss) attributable to Aon shareholders	(519,000,000)	605,000,000	(317,000,000)	(44,000,000)
Comprehensive income (loss) attributable to Aon shareholders	$ (1,195,000,000)	$ 67,000,000	$ (2,363,000,000)	$ (1,974,000,000)
7.375% Debt securities due December 2012
Financial Information for Subsidiary Guarantors and Non Guarantors
Debt Instrument, Variable Rate (as a percent)	7.375%		7.375%
3.50% senior notes due September 2015
Financial Information for Subsidiary Guarantors and Non Guarantors
Debt Instrument, Variable Rate (as a percent)	3.50%		3.50%
3.125% Senior notes due 2016
Financial Information for Subsidiary Guarantors and Non Guarantors
Debt Instrument, Variable Rate (as a percent)	3.125%		3.125%
5.00% Senior notes due September 2020
Financial Information for Subsidiary Guarantors and Non Guarantors
Debt Instrument, Variable Rate (as a percent)	5.00%		5.00%
8.205% Junior subordinated deferrable interest debentures due January 2027
Financial Information for Subsidiary Guarantors and Non Guarantors
Debt Instrument, Variable Rate (as a percent)	8.205%		8.205%
6.25% Senior notes due September 2040
Financial Information for Subsidiary Guarantors and Non Guarantors
Debt Instrument, Variable Rate (as a percent)	6.25%		6.25%