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Guarantee of Registered Securities - Narrative (Details) - subsidiary
Sep. 30, 2019
May 02, 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%
3.75% Senior Notes Due May 2, 2029 | Senior Notes        
Condensed Financial Statements, Captions [Line Items]        
Debt interest rate percentage (as a percent) 3.75% 3.75%    
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%