Fair value disclosures - Assets and liabilities measured on a recurring basis at fair value (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Assets and liabilities measured on a recurring basis at fair value
Debt securities	$ 3,797	$ 3,164
Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	3,797	3,164
Equity securities	438	441
Total assets	4,451	3,948
Total liabilities	151	195
Recurring basis | Foreign exchange contracts
Assets and liabilities measured on a recurring basis at fair value
Derivate financial instruments - assets/liabilities	207	328
Recurring basis | Commodity contracts
Assets and liabilities measured on a recurring basis at fair value
Derivate financial instruments - assets/liabilities	9	15
Recurring basis | Interest rate contracts
Assets and liabilities measured on a recurring basis at fair value
Derivate financial instruments - assets/liabilities	(151)	(195)
U.S. treasury bonds
Assets and liabilities measured on a recurring basis at fair value
Debt securities	10	9
U.S. treasury bonds | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	10	9
Other U.S. and non-U.S. government bonds
Assets and liabilities measured on a recurring basis at fair value
Debt securities	60	55
Other U.S. and non-U.S. government bonds | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	60	55
Corporate bonds and other debt securities
Assets and liabilities measured on a recurring basis at fair value
Debt securities	2,995	2,466
Corporate bonds and other debt securities | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	2,995	2,466
Asset-backed securities
Assets and liabilities measured on a recurring basis at fair value
Debt securities	192	182
Asset-backed securities | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	192	182
U.S. governmental agency | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	410	333
Residential
Assets and liabilities measured on a recurring basis at fair value
Debt securities	2	2
Residential | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	2	2
Commercial
Assets and liabilities measured on a recurring basis at fair value
Debt securities	128	117
Commercial | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	128	117
Large capitalization value | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Equity securities	223	203
Smaller company growth | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Equity securities	35	31
REIT | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Equity securities	180	207
Level 1 | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	10	9
Equity securities	258	234
Total assets	268	243
Total liabilities	0	0
Level 1 | Recurring basis | Foreign exchange contracts
Assets and liabilities measured on a recurring basis at fair value
Derivate financial instruments - assets/liabilities	0	0
Level 1 | Recurring basis | Commodity contracts
Assets and liabilities measured on a recurring basis at fair value
Derivate financial instruments - assets/liabilities	0	0
Level 1 | Recurring basis | Interest rate contracts
Assets and liabilities measured on a recurring basis at fair value
Derivate financial instruments - assets/liabilities	0	0
Level 1 | U.S. treasury bonds | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	10	9
Level 1 | Other U.S. and non-U.S. government bonds | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	0	0
Level 1 | Corporate bonds and other debt securities | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	0	0
Level 1 | Asset-backed securities | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	0	0
Level 1 | U.S. governmental agency | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	0	0
Level 1 | Residential | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	0	0
Level 1 | Commercial | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	0	0
Level 1 | Large capitalization value | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Equity securities	223	203
Level 1 | Smaller company growth | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Equity securities	35	31
Level 1 | REIT | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Equity securities	0	0
Level 2 | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	3,787	3,105
Equity securities	0	0
Total assets	4,003	3,448
Total liabilities	151	195
Level 2 | Recurring basis | Foreign exchange contracts
Assets and liabilities measured on a recurring basis at fair value
Derivate financial instruments - assets/liabilities	207	328
Level 2 | Recurring basis | Commodity contracts
Assets and liabilities measured on a recurring basis at fair value
Derivate financial instruments - assets/liabilities	9	15
Level 2 | Recurring basis | Interest rate contracts
Assets and liabilities measured on a recurring basis at fair value
Derivate financial instruments - assets/liabilities	(151)	(195)
Level 2 | U.S. treasury bonds | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	0	0
Level 2 | Other U.S. and non-U.S. government bonds | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	60	55
Level 2 | Corporate bonds and other debt securities | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	2,995	2,416
Level 2 | Asset-backed securities | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	192	182
Level 2 | U.S. governmental agency | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	410	333
Level 2 | Residential | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	2	2
Level 2 | Commercial | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	128	117
Level 2 | Large capitalization value | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Equity securities	0	0
Level 2 | Smaller company growth | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Equity securities	0	0
Level 2 | REIT | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Equity securities	0	0
Level 3 | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	0	50
Equity securities	0	0
Total assets	0	50
Total liabilities	0	0
Level 3 | Recurring basis | Foreign exchange contracts
Assets and liabilities measured on a recurring basis at fair value
Derivate financial instruments - assets/liabilities	0	0
Level 3 | Recurring basis | Commodity contracts
Assets and liabilities measured on a recurring basis at fair value
Derivate financial instruments - assets/liabilities	0	0
Level 3 | Recurring basis | Interest rate contracts
Assets and liabilities measured on a recurring basis at fair value
Derivate financial instruments - assets/liabilities	0	0
Level 3 | U.S. treasury bonds | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	0	0
Level 3 | Other U.S. and non-U.S. government bonds | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	0	0
Level 3 | Corporate bonds and other debt securities | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	0	50
Level 3 | Asset-backed securities | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	0	0
Level 3 | U.S. governmental agency | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	0	0
Level 3 | Residential | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	0	0
Level 3 | Commercial | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	0	0
Level 3 | Large capitalization value | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Equity securities	0	0
Level 3 | Smaller company growth | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Equity securities	0	0
Level 3 | REIT | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Equity securities	0	0
Measured at NAV | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	0	0
Equity securities	180	207
Total assets	180	207
Total liabilities	0	0
Measured at NAV | Recurring basis | Foreign exchange contracts
Assets and liabilities measured on a recurring basis at fair value
Derivate financial instruments - assets/liabilities	0	0
Measured at NAV | Recurring basis | Commodity contracts
Assets and liabilities measured on a recurring basis at fair value
Derivate financial instruments - assets/liabilities	0	0
Measured at NAV | Recurring basis | Interest rate contracts
Assets and liabilities measured on a recurring basis at fair value
Derivate financial instruments - assets/liabilities	0	0
Measured at NAV | U.S. treasury bonds | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	0	0
Measured at NAV | Other U.S. and non-U.S. government bonds | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	0	0
Measured at NAV | Corporate bonds and other debt securities | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	0	0
Measured at NAV | Asset-backed securities | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	0	0
Measured at NAV | U.S. governmental agency | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	0	0
Measured at NAV | Residential | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	0	0
Measured at NAV | Commercial | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	0	0
Measured at NAV | Large capitalization value | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Equity securities	0	0
Measured at NAV | Smaller company growth | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Equity securities	0	0
Measured at NAV | REIT | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Equity securities	$ 180	$ 207