Guarantee of Registered Securities - Narrative (Details)	Dec. 31, 2014	Nov. 21, 2013
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]
Debt interest rate	3.50%
3.125% Senior Notes Percent Due 2016 [Member]
Condensed Financial Statements, Captions [Line Items]
Debt interest rate	3.125%
5.00% Senior notes due September 2020 [Member]
Condensed Financial Statements, Captions [Line Items]
Debt interest rate	5.00%
8.205% Junior Subordinated Deferrable Interest Debentures Percent Due 2027 [Member]
Condensed Financial Statements, Captions [Line Items]
Debt interest rate	8.205%
6.25% Senior notes due September 2040 [Member]
Condensed Financial Statements, Captions [Line Items]
Debt interest rate	6.25%
4.250% Notes Due 2042 [Member]
Condensed Financial Statements, Captions [Line Items]
Debt interest rate	4.25%
4.45% Notes Due at 2043 [Member]
Condensed Financial Statements, Captions [Line Items]
Debt interest rate	4.45%
4.00% senior notes dues November 2023 [Member]
Condensed Financial Statements, Captions [Line Items]
Debt interest rate	4.00%	4.00%
2.875% notes due 2026 [Member]
Condensed Financial Statements, Captions [Line Items]
Debt interest rate	2.875%
3.50% Notes due June 2024 [Member]
Condensed Financial Statements, Captions [Line Items]
Debt interest rate	3.50%
4.60% notes due May 2044 [Member]
Condensed Financial Statements, Captions [Line Items]
Debt interest rate	4.60%
3.50% notes due 2024 [Member]
Condensed Financial Statements, Captions [Line Items]
Debt interest rate	3.50%
4.60% notes due 2044 [Member]
Condensed Financial Statements, Captions [Line Items]
Debt interest rate	4.60%