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Investment Information - Net realized gains and losses (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Available for sale securities:
Available for sale securities, gross gains on investment sales		$ 296	$ 259	$ 116
Available for sale securities, gross losses on investment sales		(271)	(354)	(547)
Equity securities, at fair value :
Net realized gains (losses) on securities sold		84	62	61
Net unrealized gains (losses) on equity securities still held at reporting date		130	108	88
Allowance for credit losses:
Allowance for credit losses - investment related		(6)	0	3
Allowance for credit losses - underwriting related		3	5	(1)
Derivative instruments		327	8	59
Other	[1]	(169)	251	10
Net realized gains (losses)		464	197	(165)
Fixed maturities
Available for sale securities:
Change in fair value of assets and liabilities accounted for using the fair value option		29	3	18
Other investments
Available for sale securities:
Change in fair value of assets and liabilities accounted for using the fair value option		38	(144)	27
Equity securities
Available for sale securities:
Change in fair value of assets and liabilities accounted for using the fair value option		0	(1)	1
Short-term investments
Available for sale securities:
Change in fair value of assets and liabilities accounted for using the fair value option		$ 3	$ 0	$ 0
[1] Amounts in the 2025 periods primarily include losses related to the sale of certain alternative investments accounted for under the equity method, while amounts in the 2024 period include benefits from the sale of Castel Underwriting Agencies Limited and the acquisition of RMIC Companies, Inc

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