Fair value measurements (Financial Instruments Measured At Fair Value On Recurring Basis) (Details) - USD ($) $ in Millions	Jun. 30, 2025		Dec. 31, 2024		Jun. 30, 2024
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Debt Securities, Available-for-Sale	$ 116,119		$ 110,363
Equity securities	9,913		9,151
Short-term investments	4,508		5,142
Other investments	9,032		8,597
Securities lending collateral	1,941		1,445
Separate account assets	6,481		6,231		$ 5,834
Market risk benefits	609		607
Private equities	16,313		14,769
Reserved alternative investment fund
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Equity securities	5,044		4,978
Other Investments
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Other investments	1,434		1,241
Investment Funds Limited Partnerships Partially Owned Investment Companies Fair Value
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Private equities	16,313		14,769
U.S. and local government securities
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Debt Securities, Available-for-Sale	4,033		4,070
Non-U.S.
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Debt Securities, Available-for-Sale	38,463		35,838
Corporate and asset-backed securities
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Debt Securities, Available-for-Sale	45,126		43,207
Mortgage-backed securities
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Debt Securities, Available-for-Sale	28,497		27,248
Level 1 | Fair Value, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Debt Securities, Available-for-Sale	1,783		1,765
Equity securities	4,744	[1]	4,053	[2]
Short-term investments	2,666		3,156
Other investments	599	[3]	573	[4]
Securities lending collateral	0		0
Investment derivatives	51		41
Derivatives designated as hedging instruments	0		0
Other derivative instruments	7		35
Separate account assets	6,399		6,165
Total assets measured at fair value	16,249	[1],[3],[5]	15,788	[2],[4],[6]
Investment derivatives	143		303
Derivatives designated as hedging instruments	0		0
Other derivative instruments, liability	31
Market risk benefits	0	[7]	0	[8]
Total liabilities measured at fair value	174		303
Level 1 | U.S. and local government securities | Fair Value, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Debt Securities, Available-for-Sale	1,783		1,765
Level 1 | Non-U.S. | Fair Value, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Debt Securities, Available-for-Sale	0		0
Level 1 | Corporate and asset-backed securities | Fair Value, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Debt Securities, Available-for-Sale	0		0
Level 1 | Mortgage-backed securities | Fair Value, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Debt Securities, Available-for-Sale	0		0
Level 2 | Fair Value, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Debt Securities, Available-for-Sale	110,552		105,100
Equity securities	0	[1]	0	[2]
Short-term investments	1,800		1,972
Other investments	6,999	[3]	6,783	[4]
Securities lending collateral	1,941		1,445
Investment derivatives	0		0
Derivatives designated as hedging instruments	257		146
Other derivative instruments	0		0
Separate account assets	82		66
Total assets measured at fair value	121,631	[1],[3],[5]	115,512	[2],[4],[6]
Investment derivatives	0		0
Derivatives designated as hedging instruments	223		116
Other derivative instruments, liability	5		2
Market risk benefits	0	[7]	0	[8]
Total liabilities measured at fair value	228		118
Level 2 | U.S. and local government securities | Fair Value, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Debt Securities, Available-for-Sale	2,250		2,305
Level 2 | Non-U.S. | Fair Value, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Debt Securities, Available-for-Sale	37,865		35,234
Level 2 | Corporate and asset-backed securities | Fair Value, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Debt Securities, Available-for-Sale	41,940		40,316
Level 2 | Mortgage-backed securities | Fair Value, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Debt Securities, Available-for-Sale	28,497		27,245
Level 3 | Fair Value, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Debt Securities, Available-for-Sale	3,784		3,498
Equity securities	125	[1]	120	[2]
Short-term investments	42		14
Other investments	0	[3]	0	[4]
Securities lending collateral	0		0
Investment derivatives	0		0
Derivatives designated as hedging instruments	0		0
Other derivative instruments	0		0
Separate account assets	0		0
Total assets measured at fair value	3,951	[1],[3],[5]	3,632	[2],[4],[6]
Investment derivatives	0		0
Derivatives designated as hedging instruments	0		0
Other derivative instruments, liability	0		0
Market risk benefits	609	[7]	607	[8]
Total liabilities measured at fair value	609		607
Level 3 | U.S. and local government securities | Fair Value, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Debt Securities, Available-for-Sale	0		0
Level 3 | Non-U.S. | Fair Value, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Debt Securities, Available-for-Sale	598		604
Level 3 | Corporate and asset-backed securities | Fair Value, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Debt Securities, Available-for-Sale	3,186		2,891
Level 3 | Mortgage-backed securities | Fair Value, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Debt Securities, Available-for-Sale	0		3
Total Level 1, Level 2, and Level 3 | Fair Value, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Debt Securities, Available-for-Sale	116,119		110,363
Equity securities	4,869	[1]	4,173	[2]
Short-term investments	4,508		5,142
Other investments	7,598	[3]	7,356	[4]
Securities lending collateral	1,941		1,445
Investment derivatives	51		41
Derivatives designated as hedging instruments	257		146
Other derivative instruments	7		35
Separate account assets	6,481		6,231
Total assets measured at fair value	141,831	[1],[3],[5]	134,932	[2],[4],[6]
Investment derivatives	143		303
Derivatives designated as hedging instruments	223		116
Other derivative instruments, liability	36		2
Market risk benefits	609	[7]	607	[8]
Total liabilities measured at fair value	1,011		1,028
Total Level 1, Level 2, and Level 3 | U.S. and local government securities | Fair Value, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Debt Securities, Available-for-Sale	4,033		4,070
Total Level 1, Level 2, and Level 3 | Non-U.S. | Fair Value, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Debt Securities, Available-for-Sale	38,463		35,838
Total Level 1, Level 2, and Level 3 | Corporate and asset-backed securities | Fair Value, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Debt Securities, Available-for-Sale	45,126		43,207
Total Level 1, Level 2, and Level 3 | Mortgage-backed securities | Fair Value, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Debt Securities, Available-for-Sale	$ 28,497		$ 27,248

[1] Excluded from the table above is a fund of $5,044 million, measured using NAV as a practical expedient.

[2] Excluded from the table above is a fund of $4,978 million, measured using NAV as a practical expedient.

[3] Excluded from the table above are other investments of $1,434 million, principally policy loans, measured using NAV as a practical expedient.

[4] Excluded from the table above are other investments of $1,241 million, principally policy loans, measured using NAV as a practical expedient.

[5] Excluded from the table above are private equities of $16,313 million, measured using NAV as a practical expedient.

[6] Excluded from the table above are private equities of $14,769 million, measured using NAV as a practical expedient.

[7] Refer to Note 11 for additional information on Market risk benefits.

[8] Refer to Note 11 for additional information on Market risk benefits.