Guarantee of Registered Securities (Narrative) (Details)	Mar. 31, 2014
Aon plc [Member]
Condensed Financial Statements, Captions [Line Items]
Parent company's percentage ownership of guarantors	100.00%
3.50% senior notes due September 2015 [Member]
Condensed Financial Statements, Captions [Line Items]
Stated interest rate	3.50%
3.125% Senior notes due 2016 [Member]
Condensed Financial Statements, Captions [Line Items]
Stated interest rate	3.125%
5.00% Senior notes due September 2020 [Member]
Condensed Financial Statements, Captions [Line Items]
Stated interest rate	5.00%
8.205% Junior subordinated deferrable interest debentures due January 2027 [Member]
Condensed Financial Statements, Captions [Line Items]
Stated interest rate	8.205%
6.25% Senior notes due September 2040 [Member]
Condensed Financial Statements, Captions [Line Items]
Stated interest rate	6.25%
4.250% Senior notes due 2042 [Member]
Condensed Financial Statements, Captions [Line Items]
Stated interest rate	4.25%
4.45% notes due 2043 [Member]
Condensed Financial Statements, Captions [Line Items]
Stated interest rate	4.45%
4.00% notes due 2023 [Member]
Condensed Financial Statements, Captions [Line Items]
Stated interest rate	4.00%