Fair value measurements (Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation) (Details) - Level 3 - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of year, assets	$ 90	$ 77	$ 73
Transfers into Level 3, assets	0	1	0
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	(7)	15	8
Purchased, assets	24	9	21
Sales, assets	(20)	(12)	(25)
Settlements, assets	0	0	0
Balance- End of year, assets	87	90	77
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	(7)	14	5
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	7	5
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	(1)	0	(1)
Net Realized Gains/Losses, Assets	(1)	(2)	0
Purchased, assets	5	3	9
Sales, assets	(3)	0	0
Settlements, assets	0	(5)	(6)
Balance- End of year, assets	3	3	7
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	0	(1)	0
Change in Net Unrealized Gains/Losses included in OCI at the Balance Sheet Date, Assets	0	0	0
Other investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of year, assets		0	10
Transfers into Level 3, assets			0
Transfers out of Level 3, assets			(10)
Change in Net Unrealized Gains (Losses) included in OCI, Assets			0
Net Realized Gains/Losses, Assets			0
Purchased, assets			0
Sales, assets			0
Settlements, assets			0
Balance- End of year, assets			0
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets			0
Change in Net Unrealized Gains/Losses included in OCI at the Balance Sheet Date, Assets			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	564	633	546
Transfers into Level 3, assets	21	23	24
Transfers out of Level 3, assets	(22)	(23)	(11)
Change in Net Unrealized Gains (Losses) included in OCI, Assets	13	(53)	(30)
Net Realized Gains/Losses, Assets	(4)	(6)	(1)
Purchased, assets	258	156	275
Sales, assets	(82)	(59)	(48)
Settlements, assets	(56)	(107)	(122)
Balance- End of year, assets	692	564	633
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	(1)	(2)	0
Change in Net Unrealized Gains/Losses included in OCI at the Balance Sheet Date, Assets	7	(53)	(25)
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	2,449	2,049	1,573
Transfers into Level 3, assets	30	47	91
Transfers out of Level 3, assets	(26)	(97)	(76)
Change in Net Unrealized Gains (Losses) included in OCI, Assets	28	(80)	15
Net Realized Gains/Losses, Assets	(17)	(14)	(2)
Purchased, assets	681	921	1,154
Sales, assets	(81)	(85)	(99)
Settlements, assets	(442)	(292)	(607)
Balance- End of year, assets	2,622	2,449	2,049
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date, Assets	(5)	(9)	3
Change in Net Unrealized Gains/Losses included in OCI at the Balance Sheet Date, Assets	12	(84)	17
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	11	26	60
Transfers into Level 3, assets	0	0	0
Transfers out of Level 3, assets	(15)	(9)	(18)
Change in Net Unrealized Gains (Losses) included in OCI, Assets	0	0	0
Net Realized Gains/Losses, Assets	0	0	0
Purchased, assets	15	4	18
Sales, assets	0	0	(1)
Settlements, assets	(4)	(10)	(33)
Balance- End of year, assets	7	11	26
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

Amount of unrealized gain (loss) recognized in other comprehensive income (OCI) from asset measured at fair value on recurring basis using unobservable input (level 3) and still held.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482106/820-10-50-2

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Amount of gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3).

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-Topic 820
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Amount of gain (loss) recognized in other comprehensive income (OCI) from asset measured at fair value on recurring basis using unobservable input (level 3).

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Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy.

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Amount of transfers of financial instrument classified as an asset out of level 3 of the fair value hierarchy.

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