Fair value measurements (Financial Instruments Measured At Fair Value On Recurring Basis) (Details) - USD ($) $ in Millions	Mar. 31, 2025		Dec. 31, 2024		Mar. 31, 2024
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Debt Securities, Available-for-Sale	$ 111,123		$ 110,363
Equity securities	9,556		9,151
Short-term investments	4,432		5,142
Other investments	9,224		8,597
Securities lending collateral	1,909		1,445
Separate account assets	6,285		6,231		$ 5,864
Market risk benefits	708		607
Private equities	15,506		14,769
Reserved alternative investment fund
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Equity securities	5,031		4,978
Other Investments
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Other investments	1,329		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	15,506		14,769
U.S. and local government securities
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Debt Securities, Available-for-Sale	4,025		4,070
Non-U.S.
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Debt Securities, Available-for-Sale	36,302		35,838
Corporate and asset-backed securities
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Debt Securities, Available-for-Sale	43,280		43,207
Mortgage-backed securities
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Debt Securities, Available-for-Sale	27,516		27,248
Level 1 | Fair Value, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Debt Securities, Available-for-Sale	1,739		1,765
Equity securities	4,412	[1]	4,053	[2]
Short-term investments	2,484		3,156
Other investments	577	[3]	573	[4]
Securities lending collateral	0		0
Investment derivatives	40		41
Derivatives designated as hedging instruments	0		0
Other derivative instruments	5		35
Separate account assets	6,212		6,165
Total assets measured at fair value	15,469	[1],[3],[5]	15,788	[2],[4],[6]
Investment derivatives	294		303
Derivatives designated as hedging instruments	0		0
Other derivative instruments, liability	0
Market risk benefits	0	[7]	0	[8]
Total liabilities measured at fair value	294		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	1,739		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	105,780		105,100
Equity securities	0	[1]	0	[2]
Short-term investments	1,930		1,972
Other investments	7,318	[3]	6,783	[4]
Securities lending collateral	1,909		1,445
Investment derivatives	0		0
Derivatives designated as hedging instruments	132		146
Other derivative instruments	0		0
Separate account assets	73		66
Total assets measured at fair value	117,142	[1],[3],[5]	115,512	[2],[4],[6]
Investment derivatives	0		0
Derivatives designated as hedging instruments	101		116
Other derivative instruments, liability	5		2
Market risk benefits	0	[7]	0	[8]
Total liabilities measured at fair value	106		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	2,286		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	35,715		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	40,263		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	27,516		27,245
Level 3 | Fair Value, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Debt Securities, Available-for-Sale	3,604		3,498
Equity securities	113	[1]	120	[2]
Short-term investments	18		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	3,735	[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	708	[7]	607	[8]
Total liabilities measured at fair value	708		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	587		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	3,017		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	111,123		110,363
Equity securities	4,525	[1]	4,173	[2]
Short-term investments	4,432		5,142
Other investments	7,895	[3]	7,356	[4]
Securities lending collateral	1,909		1,445
Investment derivatives	40		41
Derivatives designated as hedging instruments	132		146
Other derivative instruments	5		35
Separate account assets	6,285		6,231
Total assets measured at fair value	136,346	[1],[3],[5]	134,932	[2],[4],[6]
Investment derivatives	294		303
Derivatives designated as hedging instruments	101		116
Other derivative instruments, liability	5		2
Market risk benefits	708	[7]	607	[8]
Total liabilities measured at fair value	1,108		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	4,025		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	36,302		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	43,280		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	$ 27,516		$ 27,248

[1] Excluded from the table above is a fund of $5,031 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 $1,329 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 $15,506 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 10 for additional information on Market risk benefits.

[8] Refer to Note 10 for additional information on Market risk benefits.