Fair value measurements (Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation) (Details) - Level 3 - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of year, assets	$ 87	$ 90	$ 77
Transfers into Level 3, assets	0	0	1
Transfers out of Level 3, assets	0	0	0
Change in Net Unrealized Gains (Losses) included in OCI, Assets	0	0	0
Net Realized Gains/Losses, Assets	8	(7)	15
Purchased, assets	43	24	9
Sales, assets	(18)	(20)	(12)
Settlements, assets	0	0	0
Balance- End of year, assets	120	87	90
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	7	(7)	14
Change in Net Unrealized Gains/Losses included in OCI at the Balance Sheet Date, Assets	0	0	0
Short-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of year, assets	3	3	7
Transfers into Level 3, assets	0	0	0
Transfers out of Level 3, assets	0	0	0
Change in Net Unrealized Gains (Losses) included in OCI, Assets	0	(1)	0
Net Realized Gains/Losses, Assets	0	(1)	(2)
Purchased, assets	20	5	3
Sales, assets	(1)	(3)	0
Settlements, assets	(8)	0	(5)
Balance- End of year, assets	14	3	3
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	0	0	(1)
Change in Net Unrealized Gains/Losses included in OCI at the Balance Sheet Date, Assets	(1)	0	0
Available-for-sale Securities [Member] | Non-U.S.
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of year, assets	692	564	633
Transfers into Level 3, assets	2	21	23
Transfers out of Level 3, assets	(7)	(22)	(23)
Change in Net Unrealized Gains (Losses) included in OCI, Assets	7	13	(53)
Net Realized Gains/Losses, Assets	(13)	(4)	(6)
Purchased, assets	262	258	156
Sales, assets	(99)	(82)	(59)
Settlements, assets	(240)	(56)	(107)
Balance- End of year, assets	604	692	564
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	0	(1)	(2)
Change in Net Unrealized Gains/Losses included in OCI at the Balance Sheet Date, Assets	(2)	7	(53)
Available-for-sale Securities [Member] | Corporate and asset-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of year, assets	2,622	2,449	2,049
Transfers into Level 3, assets	57	30	47
Transfers out of Level 3, assets	(9)	(26)	(97)
Change in Net Unrealized Gains (Losses) included in OCI, Assets	12	28	(80)
Net Realized Gains/Losses, Assets	(15)	(17)	(14)
Purchased, assets	1,042	681	921
Sales, assets	(250)	(81)	(85)
Settlements, assets	(568)	(442)	(292)
Balance- End of year, assets	2,891	2,622	2,449
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	(3)	(5)	(9)
Change in Net Unrealized Gains/Losses included in OCI at the Balance Sheet Date, Assets	(2)	12	(84)
Available-for-sale Securities [Member] | Mortgage-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of year, assets	7	11	26
Transfers into Level 3, assets	0	0	0
Transfers out of Level 3, assets	(54)	(15)	(9)
Change in Net Unrealized Gains (Losses) included in OCI, Assets	0	0	0
Net Realized Gains/Losses, Assets	0	0	0
Purchased, assets	54	15	4
Sales, assets	0	0	0
Settlements, assets	(4)	(4)	(10)
Balance- End of year, assets	3	7	11
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	0	0	0
Change in Net Unrealized Gains/Losses included in OCI at the Balance Sheet Date, Assets	$ 0	$ 0	$ 0