Fair value measurements (Assets, Measured At Fair Value Using Significant Unobservable Inputs) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Other comprehensive income (loss), before income tax	Other comprehensive income (loss), before income tax	Other comprehensive income (loss), before income tax	Other comprehensive income (loss), before income tax
Fair Value, Asset (Liability), Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Total revenues	Total revenues	Total revenues	Total revenues
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Total revenues	Total revenues	Total revenues	Total revenues
Level 3 | Equity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	$ 100	$ 86	$ 87	$ 90
Transfers Into Level 3, Asset			0	0
Transfers out of Level 3, Assets	0	0	0	0
Change in Net Unrealized Gains (Losses) included in OCI, Assets	0	0	0	0
Net Realized Gains/ (Losses), Assets	7	0	6	(6)
Purchases, Assets	17	4	36	15
Sales, Assets	(6)	(5)	(11)	(14)
Settlements, Assets	0	0	0	0
Balance-End of Period, Assets	118	85	118	85
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	6	0	5	(6)
Change in Net Unrealized Gains/Losses included in OCI at the Balance Sheet Date, Assets	0	0	0	0
Level 3 | Short-term Investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	12	3	3	3
Transfers Into Level 3, Asset			0	0
Transfers out of Level 3, Assets	0	0	0	0
Change in Net Unrealized Gains (Losses) included in OCI, Assets	(1)	0	(2)	(1)
Net Realized Gains/ (Losses), Assets	0	0	0	0
Purchases, Assets	6	0	22	3
Sales, Assets	(1)	(1)	(1)	(3)
Settlements, Assets	(1)	0	(7)	0
Balance-End of Period, Assets	15	2	15	2
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	0	0	0	0
Change in Net Unrealized Gains/Losses included in OCI at the Balance Sheet Date, Assets	(1)	0	(2)	0
Level 3 | Available-for-Sale Debt Securities | Non-U.S.
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	632	649	692	564
Transfers Into Level 3, Asset			1	21
Transfers out of Level 3, Assets	(1)	(22)	(7)	(22)
Change in Net Unrealized Gains (Losses) included in OCI, Assets	10	(6)	18	5
Net Realized Gains/ (Losses), Assets	(1)	(2)	(5)	(3)
Purchases, Assets	59	46	214	151
Sales, Assets	(18)	(17)	(69)	(52)
Settlements, Assets	(42)	(15)	(205)	(31)
Balance-End of Period, Assets	639	633	639	633
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	0	(1)	(1)	(1)
Change in Net Unrealized Gains/Losses included in OCI at the Balance Sheet Date, Assets	11	(8)	14	1
Level 3 | Available-for-Sale Debt Securities | Corporate and asset-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	2,709	2,524	2,622	2,449
Transfers Into Level 3, Asset			5	3
Transfers out of Level 3, Assets	0	(10)	(3)	(23)
Change in Net Unrealized Gains (Losses) included in OCI, Assets	16	6	26	8
Net Realized Gains/ (Losses), Assets	(10)	(4)	(15)	(7)
Purchases, Assets	316	158	785	481
Sales, Assets	(69)	(40)	(176)	(70)
Settlements, Assets	(184)	(65)	(466)	(272)
Balance-End of Period, Assets	2,778	2,569	2,778	2,569
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	(8)	(4)	(10)	(6)
Change in Net Unrealized Gains/Losses included in OCI at the Balance Sheet Date, Assets	12	4	17	2
Level 3 | Available-for-Sale Debt Securities | Mortgage-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets	20	10	7	11
Transfers Into Level 3, Asset			0	0
Transfers out of Level 3, Assets	(14)	0	(14)	0
Change in Net Unrealized Gains (Losses) included in OCI, Assets	0	0	0	0
Net Realized Gains/ (Losses), Assets	0	0	0	0
Purchases, Assets	40	16	55	16
Sales, Assets	0	0	0	0
Settlements, Assets	0	(1)	(2)	(2)
Balance-End of Period, Assets	46	25	46	25
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	0	0	0	0
Change in Net Unrealized Gains/Losses included in OCI at the Balance Sheet Date, Assets	$ 0	$ 0	$ 0	$ 0