Guarantee of Registered Securities - Narrative (Details) - subsidiary	Mar. 31, 2019	Dec. 31, 2018	Dec. 03, 2018
Condensed Financial Statements, Captions [Line Items]
Number of subsidiaries		2
Aon plc
Condensed Financial Statements, Captions [Line Items]
Parent company's percentage ownership of guarantors	100.00%
5.00% Senior notes due September 2020
Condensed Financial Statements, Captions [Line Items]
Debt interest rate percentage (as a percent)	5.00%
8.205% Junior subordinated deferrable interest debentures due January 2027
Condensed Financial Statements, Captions [Line Items]
Debt interest rate percentage (as a percent)	8.205%
6.25% Senior notes due September 2040
Condensed Financial Statements, Captions [Line Items]
Debt interest rate percentage (as a percent)	6.25%
4.250% Senior notes due 2042
Condensed Financial Statements, Captions [Line Items]
Debt interest rate percentage (as a percent)	4.25%
4.45% notes due 2043
Condensed Financial Statements, Captions [Line Items]
Debt interest rate percentage (as a percent)	4.45%
4.00% notes due 2023
Condensed Financial Statements, Captions [Line Items]
Debt interest rate percentage (as a percent)	4.00%
2.875% notes due 2026
Condensed Financial Statements, Captions [Line Items]
Debt interest rate percentage (as a percent)	2.875%
3.50% Notes due June 2024
Condensed Financial Statements, Captions [Line Items]
Debt interest rate percentage (as a percent)	3.50%
4.60% notes due May 2044
Condensed Financial Statements, Captions [Line Items]
Debt interest rate percentage (as a percent)	4.60%
4.75% Notes Due May 2045
Condensed Financial Statements, Captions [Line Items]
Debt interest rate percentage (as a percent)	4.75%
4.500% Senior Notes Due December 2028 | Senior Notes
Condensed Financial Statements, Captions [Line Items]
Debt interest rate percentage (as a percent)	4.50%		4.50%
2.80% Senior Notes Due 2021
Condensed Financial Statements, Captions [Line Items]
Debt interest rate percentage (as a percent)	2.80%
3.875% due in December 2025
Condensed Financial Statements, Captions [Line Items]
Debt interest rate percentage (as a percent)	3.875%