Employee Benefits (Details 5) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
U.K. And non-U.S. Pension Plan
Fair values of Aon's pension plan assets
Total	$ 5,869	$ 5,098	$ 4,288
U.K. And non-U.S. Pension Plan | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair values of Aon's pension plan assets
Total	1,675	1,703
U.K. And non-U.S. Pension Plan | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalents
Fair values of Aon's pension plan assets
Total	303	276
U.K. And non-U.S. Pension Plan | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Equity Securities
Fair values of Aon's pension plan assets
Total	137	128
U.K. And non-U.S. Pension Plan | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed income securities
Fair values of Aon's pension plan assets
Total	1,235	1,299
U.K. And non-U.S. Pension Plan | Significant Other Observable Inputs (Level 2)
Fair values of Aon's pension plan assets
Total	3,082	2,496
U.K. And non-U.S. Pension Plan | Significant Other Observable Inputs (Level 2) | Equity derivatives
Fair values of Aon's pension plan assets
Total	103	11
U.K. And non-U.S. Pension Plan | Significant Other Observable Inputs (Level 2) | Global
Fair values of Aon's pension plan assets
Total	1,241	960
U.K. And non-U.S. Pension Plan | Significant Other Observable Inputs (Level 2) | Europe
Fair values of Aon's pension plan assets
Total	319	347
U.K. And non-U.S. Pension Plan | Significant Other Observable Inputs (Level 2) | North America
Fair values of Aon's pension plan assets
Total	65	56
U.K. And non-U.S. Pension Plan | Significant Other Observable Inputs (Level 2) | Fixed income investments
Fair values of Aon's pension plan assets
Total	973	823
U.K. And non-U.S. Pension Plan | Significant Other Observable Inputs (Level 2) | Fixed income derivatives
Fair values of Aon's pension plan assets
Total	23	21
U.K. And non-U.S. Pension Plan | Significant Other Observable Inputs (Level 2) | Fixed income securities
Fair values of Aon's pension plan assets
Total	63	56
U.K. And non-U.S. Pension Plan | Significant Other Observable Inputs (Level 2) | Derivatives
Fair values of Aon's pension plan assets
Total	245	178
U.K. And non-U.S. Pension Plan | Significant Other Observable Inputs (Level 2) | Commodities
Fair values of Aon's pension plan assets
Total	30	26
U.K. And non-U.S. Pension Plan | Significant Other Observable Inputs (Level 2) | REITS
Fair values of Aon's pension plan assets
Total	5	4
U.K. And non-U.S. Pension Plan | Significant Other Observable Inputs (Level 2) | Commodity derivatives
Fair values of Aon's pension plan assets
Total	15	14
U.K. And non-U.S. Pension Plan | Significant Unobservable Inputs (Level 3)
Fair values of Aon's pension plan assets
Total	1,112	899	927
U.K. And non-U.S. Pension Plan | Significant Unobservable Inputs (Level 3) | Global
Fair values of Aon's pension plan assets
Total			162
U.K. And non-U.S. Pension Plan | Significant Unobservable Inputs (Level 3) | Annuities
Fair values of Aon's pension plan assets
Total	568	419	380
U.K. And non-U.S. Pension Plan | Significant Unobservable Inputs (Level 3) | Real Estate
Fair values of Aon's pension plan assets
Total	87	134	127
U.K. And non-U.S. Pension Plan | Significant Unobservable Inputs (Level 3) | Alternative investments
Fair values of Aon's pension plan assets
Total	457	346	218
U.K. And non-U.S. Pension Plan | Fair Value
Fair values of Aon's pension plan assets
Total	5,869	5,098
U.K. And non-U.S. Pension Plan | Fair Value | Cash and cash equivalents
Fair values of Aon's pension plan assets
Total	303	276
U.K. And non-U.S. Pension Plan | Fair Value | Equity derivatives
Fair values of Aon's pension plan assets
Total	103	11
U.K. And non-U.S. Pension Plan | Fair Value | Global
Fair values of Aon's pension plan assets
Total	1,241	960
U.K. And non-U.S. Pension Plan | Fair Value | Europe
Fair values of Aon's pension plan assets
Total	319	347
U.K. And non-U.S. Pension Plan | Fair Value | North America
Fair values of Aon's pension plan assets
Total	65	56
U.K. And non-U.S. Pension Plan | Fair Value | Other Equity Securities
Fair values of Aon's pension plan assets
Total	137	128
U.K. And non-U.S. Pension Plan | Fair Value | Fixed income investments
Fair values of Aon's pension plan assets
Total	973	823
U.K. And non-U.S. Pension Plan | Fair Value | Fixed income derivatives
Fair values of Aon's pension plan assets
Total	23	21
U.K. And non-U.S. Pension Plan | Fair Value | Fixed income securities
Fair values of Aon's pension plan assets
Total	1,298	1,355
U.K. And non-U.S. Pension Plan | Fair Value | Annuities
Fair values of Aon's pension plan assets
Total	568	419
U.K. And non-U.S. Pension Plan | Fair Value | Derivatives
Fair values of Aon's pension plan assets
Total	245	178
U.K. And non-U.S. Pension Plan | Fair Value | Commodities
Fair values of Aon's pension plan assets
Total	30	26
U.K. And non-U.S. Pension Plan | Fair Value | REITS
Fair values of Aon's pension plan assets
Total	5	4
U.K. And non-U.S. Pension Plan | Fair Value | Real Estate
Fair values of Aon's pension plan assets
Total	87	134
U.K. And non-U.S. Pension Plan | Fair Value | Alternative investments
Fair values of Aon's pension plan assets
Total	457	346
U.K. And non-U.S. Pension Plan | Fair Value | Commodity derivatives
Fair values of Aon's pension plan assets
Total	15	14
U.S. Pension Plan
Fair values of Aon's pension plan assets
Total	1,631	1,325	1,244
U.S. Pension Plan | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair values of Aon's pension plan assets
Total	434	287
U.S. Pension Plan | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalents
Fair values of Aon's pension plan assets
Total	22	22
U.S. Pension Plan | Quoted Prices in Active Markets for Identical Assets (Level 1) | Large cap domestic
Fair values of Aon's pension plan assets
Total	233	151
U.S. Pension Plan | Quoted Prices in Active Markets for Identical Assets (Level 1) | Large cap international
Fair values of Aon's pension plan assets
Total	59	9
U.S. Pension Plan | Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity derivatives
Fair values of Aon's pension plan assets
Total	69	63
U.S. Pension Plan | Quoted Prices in Active Markets for Identical Assets (Level 1) | Real estate and REITS
Fair values of Aon's pension plan assets
Total	51	42
U.S. Pension Plan | Significant Other Observable Inputs (Level 2)
Fair values of Aon's pension plan assets
Total	935	847
U.S. Pension Plan | Significant Other Observable Inputs (Level 2) | Small Cap Domestic
Fair values of Aon's pension plan assets
Total	44	58
U.S. Pension Plan | Significant Other Observable Inputs (Level 2) | Large cap international
Fair values of Aon's pension plan assets
Total	129	88
U.S. Pension Plan | Significant Other Observable Inputs (Level 2) | Equity derivatives
Fair values of Aon's pension plan assets
Total	157	110
U.S. Pension Plan | Significant Other Observable Inputs (Level 2) | Corporate bonds
Fair values of Aon's pension plan assets
Total	421	355
U.S. Pension Plan | Significant Other Observable Inputs (Level 2) | Government and agency bonds
Fair values of Aon's pension plan assets
Total	97	116
U.S. Pension Plan | Significant Other Observable Inputs (Level 2) | Asset-backed securities
Fair values of Aon's pension plan assets
Total	18	18
U.S. Pension Plan | Significant Other Observable Inputs (Level 2) | Fixed income derivatives
Fair values of Aon's pension plan assets
Total	52	83
U.S. Pension Plan | Significant Other Observable Inputs (Level 2) | Commodity derivatives
Fair values of Aon's pension plan assets
Total	17	19
U.S. Pension Plan | Significant Unobservable Inputs (Level 3)
Fair values of Aon's pension plan assets
Total	262	191	193
U.S. Pension Plan | Significant Unobservable Inputs (Level 3) | Alternative investments
Fair values of Aon's pension plan assets
Total	262	191
U.S. Pension Plan | Fair Value
Fair values of Aon's pension plan assets
Total	1,631	1,325
U.S. Pension Plan | Fair Value | Cash and cash equivalents
Fair values of Aon's pension plan assets
Total	22	22
U.S. Pension Plan | Fair Value | Large cap domestic
Fair values of Aon's pension plan assets
Total	233	151
U.S. Pension Plan | Fair Value | Small Cap Domestic
Fair values of Aon's pension plan assets
Total	44	58
U.S. Pension Plan | Fair Value | Large cap international
Fair values of Aon's pension plan assets
Total	188	97
U.S. Pension Plan | Fair Value | Equity derivatives
Fair values of Aon's pension plan assets
Total	226	173
U.S. Pension Plan | Fair Value | Corporate bonds
Fair values of Aon's pension plan assets
Total	421	355
U.S. Pension Plan | Fair Value | Government and agency bonds
Fair values of Aon's pension plan assets
Total	97	116
U.S. Pension Plan | Fair Value | Asset-backed securities
Fair values of Aon's pension plan assets
Total	18	18
U.S. Pension Plan | Fair Value | Fixed income derivatives
Fair values of Aon's pension plan assets
Total	52	83
U.S. Pension Plan | Fair Value | Alternative investments
Fair values of Aon's pension plan assets
Total	262	191
U.S. Pension Plan | Fair Value | Commodity derivatives
Fair values of Aon's pension plan assets
Total	17	19
U.S. Pension Plan | Fair Value | Real estate and REITS
Fair values of Aon's pension plan assets
Total	$ 51	$ 42