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Commitments, contingencies, and guarantees (Narrative) (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Financial Instruments Owned and Pledged as Collateral [Line Items]      
Derivative Liability, Fair Value, Amount Subject to a Master Netting Agreement $ 10    
Derivative Asset, Fair Value, Amount Subject to a Master Netting Agreement   $ 1  
Securities lending collateral 1,092 1,046  
Securities lending payable 1,093 1,047  
Approximate percentage of gross premiums written that were generated from or placed by Marsh     10.00%
Purchase Commitment, Remaining Minimum Amount Committed 716    
Carrying value of limited partnerships and partially-owned investment companies included in other investments 3,400    
Funding commitments relating to limited partnerships and partially-owned investment companies 2,000    
Total rental expense related to operating leases 209 126 $ 127
Securities Sold under Agreements to Repurchase 1,403 1,404  
Equity Option [Member]      
Financial Instruments Owned and Pledged as Collateral [Line Items]      
Derivative Asset, Fair Value, Gross Liability 0    
Letter of Credit [Member]      
Financial Instruments Owned and Pledged as Collateral [Line Items]      
Line of Credit Facility, Current Borrowing Capacity 1,500    
Letter of Credit [Member] | Letter Of Credit Unsecured Expiring November 2017      
Financial Instruments Owned and Pledged as Collateral [Line Items]      
Line of Credit Facility, Capacity Available for Specific Purpose Other than for Trade Purchases 300    
Line of Credit Facility, Amount Outstanding 443    
Maturity Overnight [Member]      
Financial Instruments Owned and Pledged as Collateral [Line Items]      
Secured Borrowings, Gross, Difference, Amount $ 1 $ 1