Fair value disclosures - Assets and liabilities measured on a recurring basis at fair value (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Assets and liabilities measured on a recurring basis at fair value
Debt securities	$ 4,042	$ 3,797
Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	4,042	3,797
Equity securities	469	438
Total assets	4,628	4,451
Total liabilities	226	151
Recurring basis | Foreign exchange contracts
Assets and liabilities measured on a recurring basis at fair value
Derivate financial instruments - assets/liabilities	117	207
Recurring basis | Interest rate contracts
Assets and liabilities measured on a recurring basis at fair value
Derivate financial instruments - assets/liabilities	(191)	(151)
Recurring basis | Commodity contracts
Assets and liabilities measured on a recurring basis at fair value
Derivate financial instruments - assets/liabilities	(2)	9
Recurring basis | Total return swap contracts
Assets and liabilities measured on a recurring basis at fair value
Derivate financial instruments - assets/liabilities	(33)
U.S. treasury bonds
Assets and liabilities measured on a recurring basis at fair value
Debt securities	10	10
U.S. treasury bonds | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	10	10
Other U.S. and non-U.S. government bonds
Assets and liabilities measured on a recurring basis at fair value
Debt securities	68	60
Other U.S. and non-U.S. government bonds | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	68	60
Corporate bonds and other debt securities
Assets and liabilities measured on a recurring basis at fair value
Debt securities	3,170	2,995
Corporate bonds and other debt securities | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	3,170	2,995
Asset-backed securities
Assets and liabilities measured on a recurring basis at fair value
Debt securities	219	192
Asset-backed securities | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	219	192
U.S. governmental agency | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	443	410
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	130	128
Commercial | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	130	128
Large capitalization value | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Equity securities	261	223
Smaller company growth | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Equity securities	41	35
REIT | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Equity securities	167	180
Level 1 | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	10	10
Equity securities	302	258
Total assets	312	268
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 | Interest rate 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 | Total return swap contracts
Assets and liabilities measured on a recurring basis at fair value
Derivate financial instruments - assets/liabilities	0
Level 1 | U.S. treasury bonds | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	10	10
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	261	223
Level 1 | Smaller company growth | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Equity securities	41	35
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	4,032	3,787
Equity securities	0	0
Total assets	4,149	4,003
Total liabilities	226	151
Level 2 | Recurring basis | Foreign exchange contracts
Assets and liabilities measured on a recurring basis at fair value
Derivate financial instruments - assets/liabilities	117	207
Level 2 | Recurring basis | Interest rate contracts
Assets and liabilities measured on a recurring basis at fair value
Derivate financial instruments - assets/liabilities	(191)	(151)
Level 2 | Recurring basis | Commodity contracts
Assets and liabilities measured on a recurring basis at fair value
Derivate financial instruments - assets/liabilities	(2)	9
Level 2 | Recurring basis | Total return swap contracts
Assets and liabilities measured on a recurring basis at fair value
Derivate financial instruments - assets/liabilities	(33)
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	68	60
Level 2 | Corporate bonds and other debt securities | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	3,170	2,995
Level 2 | Asset-backed securities | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	219	192
Level 2 | U.S. governmental agency | Recurring basis
Assets and liabilities measured on a recurring basis at fair value
Debt securities	443	410
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	130	128
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	0
Equity securities	0	0
Total assets	0	0
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 | Interest rate 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 | Total return swap contracts
Assets and liabilities measured on a recurring basis at fair value
Derivate financial instruments - assets/liabilities	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	0
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	167	180
Total assets	167	180
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 | Interest rate 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 | Total return swap contracts
Assets and liabilities measured on a recurring basis at fair value
Derivate financial instruments - assets/liabilities	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	$ 167	$ 180