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Acquisitions and Dispositions of Businesses - Completed Acquisitions (Details)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 01, 2021
USD ($)
Sep. 30, 2021
acquisition
Sep. 30, 2020
acquisition
Sep. 30, 2021
acquisition
Sep. 30, 2020
acquisition
Apr. 06, 2020
Jan. 31, 2020
Jan. 03, 2020
Jan. 01, 2020
Business Acquisition [Line Items]                  
Number of business acquired under business combination | acquisition   1 0 1 6        
Aon India Insurance Brokers Limited                  
Business Acquisition [Line Items]                  
Percentage of capital acquired 51.00%                
Cash consideration to acquire businesses $ 15                
Equity interest in acquiree, percentage 49.00%                
Equity interest in acquiree $ 15                
Consideration transferred $ 30                
Farmington Administrative Services LLC                  
Business Acquisition [Line Items]                  
Percentage of capital acquired           100.00%      
Cytelligence Inc.                  
Business Acquisition [Line Items]                  
Percentage of capital acquired             100.00%    
Cover Wallet Inc.                  
Business Acquisition [Line Items]                  
Percentage of capital acquired               100.00%  
TRIUM GmbH Insurance Broker                  
Business Acquisition [Line Items]                  
Percentage of capital acquired                 100.00%
Assimedia SA                  
Business Acquisition [Line Items]                  
Percentage of capital acquired                 100.00%
Apollo Conseil et Courtage                  
Business Acquisition [Line Items]                  
Percentage of capital acquired                 100.00%