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Commitments, contingencies, and guarantees (Narrative) (Detail) - USD ($)
$ in Millions
1 Months Ended 12 Months Ended
Dec. 31, 2019
May 31, 2019
Dec. 31, 2021
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Jan. 31, 2020
Oct. 25, 2017
Financial Instruments Owned and Pledged as Collateral [Line Items]                
Concentration Risk, Percentage           10.00%    
Derivative Liability, Fair Value, Amount Subject to a Master Netting Agreement $ 75     $ 75 $ 95      
Securities Sold under Agreements to Repurchase $ 1,416     $ 1,416 $ 1,418      
Concentration Risk Percentage Marsh 12.00%     12.00% 10.00%      
Purchase Commitment, Remaining Minimum Amount Committed $ 731     $ 731        
Carrying value of limited partnerships and partially-owned investment companies included in other investments 4,700     4,700        
Funding commitments relating to limited partnerships and partially-owned investment companies 3,300     3,300        
Line of Credit Facility, Current Borrowing Capacity               $ 1,000
Line of Credit Facility, Maximum Borrowing Capacity 2,000     2,000        
Line of Credit Facility, Capacity Available for Specific Purpose Other than for Trade Purchases 1,000     1,000        
Operating Lease, Right-of-Use Asset 551     551        
Operating Lease, Liability $ 603     $ 603        
Operating Lease, Weighted Average Remaining Lease Term 5 years 4 months 24 days     5 years 4 months 24 days        
Operating Lease, Weighted Average Discount Rate, Percent 2.70%     2.70%        
Total rental expense related to operating leases       $ 171 $ 169 $ 211    
Deposit Assets $ 93     93 $ 97      
Letter of Credit [Member]                
Financial Instruments Owned and Pledged as Collateral [Line Items]                
Line of Credit Facility, Amount Outstanding $ 567     $ 567        
Huatai Group [Member]                
Financial Instruments Owned and Pledged as Collateral [Line Items]                
Equity method investment, additional ownership percentage obtained 4.70%              
Ownership Percentage 30.90%     30.90% 20.00%      
CHINA | Huatai Group [Member]                
Financial Instruments Owned and Pledged as Collateral [Line Items]                
Equity method investment, additional ownership percentage obtained   6.20%            
Ownership Percentage 31.00%     31.00% 20.00%      
CHINA | Subsequent Event [Member] | Huatai Group [Member]                
Financial Instruments Owned and Pledged as Collateral [Line Items]                
Equity method investment, additional 22.4 ownership percentage obtained     22.40%          
Deposit Assets             $ 1,550