Commitments, contingencies, and guarantees (Narrative) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Financial Instruments Owned and Pledged as Collateral [Line Items]
Derivative Liability, Fair Value, Amount Subject to a Master Netting Agreement	$ 123	$ 30
Securities Sold under Agreements to Repurchase	$ 1,406	$ 1,405
Concentration Risk Percentage Marsh	12.00%	12.00%	12.00%
Purchase Commitment, Remaining Minimum Amount Committed	$ 771
Carrying value of limited partnerships and partially-owned investment companies included in other investments	9,800	$ 6,500
Funding commitments relating to limited partnerships and partially-owned investment companies	7,200	3,200
Line of Credit Facility, Current Borrowing Capacity	3,700
Line of Credit Facility, Capacity Available for Specific Purpose Other than for Trade Purchases	1,900
Operating Lease, Right-of-Use Asset	445	473
Operating Lease, Liability	$ 484	517
Operating Lease, Weighted Average Remaining Lease Term	4 years 10 months 24 days
Operating Lease, Weighted Average Discount Rate, Percent	2.10%
Deposit Assets	$ 101	$ 107
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible Enumeration]
Operating Lease, Liability, Statement of Financial Position [Extensible List]	Other Liabilities	Other Liabilities
Repurchase agreements	$ 1,406	$ 1,405
Operating Lease, Cost	149	$ 152	$ 171
Lessee, Lease Not Yet Commenced, Noncurrent Amount	650
Commitment To Purchase Underlying Portfolio Investment
Financial Instruments Owned and Pledged as Collateral [Line Items]
Purchase Commitment, Remaining Minimum Amount Committed	500
Letter of Credit [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Line of Credit Facility, Amount Outstanding	$ 1,400