Commitments, contingencies, and guarantees (Narrative) (Detail) € in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2023 EUR (€)
Financial Instruments Owned and Pledged as Collateral [Line Items]
Derivative Liability, Fair Value, Amount Subject to a Master Netting Agreement	$ 115	$ 60
Securities Sold under Agreements to Repurchase	2,833	1,419
Repurchase agreements (includes VIE balances of $1,009 and nil)	$ 2,833	$ 1,419
Concentration Risk Percentage Marsh	11.00%	11.00%	12.00%	11.00%
Purchase Commitment, Remaining Minimum Amount Committed	$ 1,000	$ 770
Carrying value of limited partnerships and partially-owned investment companies included in other investments	13,900	12,000
Funding commitments relating to limited partnerships and partially-owned investment companies	6,200	7,400
Line of Credit Facility, Current Borrowing Capacity	4,000
Line of Credit Facility, Capacity Available for Specific Purpose Other than for Trade Purchases	3,000
Operating Lease, Right-of-Use Asset	784	607
Operating Lease, Liability	$ 832	$ 633
Operating Lease, Weighted Average Remaining Lease Term	11 years 9 months 18 days			11 years 9 months 18 days
Operating Lease, Weighted Average Discount Rate, Percent	4.60%			4.60%
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		Other liabilities
Operating Lease, Cost	$ 181	$ 161	$ 149
Lessee, Lease Not Yet Commenced, Noncurrent Amount	621
Interest expense	672	570	492
Other Comprehensive Income (Loss), before Tax	3,678	(10,076)	$ (2,507)
Long-Term Debt [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Hedged Liability, Fair Value Hedge	$ 1,600			€ 1,500
Revenue Benchmark
Financial Instruments Owned and Pledged as Collateral [Line Items]
Concentration Risk, Customer	accounted for more than 10 percent
Letter of Credit [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Line of Credit Facility, Amount Outstanding	$ 948	1,400
Fair Value Hedging
Financial Instruments Owned and Pledged as Collateral [Line Items]
Other Comprehensive Income (Loss), before Tax	67	(83)
Net Investment Hedging
Financial Instruments Owned and Pledged as Collateral [Line Items]
Other Comprehensive Income (Loss), before Tax	(71)	(57)
Fair Value Hedging
Financial Instruments Owned and Pledged as Collateral [Line Items]
Interest expense	$ (16)	$ (5)