Allowance for Expected Credit Losses - Ceded credit risk (Details) - USD ($) $ in Millions		9 Months Ended	12 Months Ended
		Sep. 30, 2025	Dec. 31, 2024	Jun. 30, 2025	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023
Ceded Credit Risk [Line Items]
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses		$ 9,070	$ 8,260	$ 9,044	$ 7,948	$ 7,473	$ 7,064
Reinsurance recoverable | Reinsurer concentration risk | AM Best A- or better
Ceded Credit Risk [Line Items]
Concentration risk percentage		63.80%	63.80%
Reinsurance recoverable | Reinsurer concentration risk | AM Best Rating Below A- | All Other Carriers
Ceded Credit Risk [Line Items]
Concentration risk percentage		0.10%	0.00%
Reinsurance recoverable | Reinsurer concentration risk | Not Rated | Ceded credit risk, secured
Ceded Credit Risk [Line Items]
Concentration risk percentage		95.00%	95.00%
Reinsurance recoverable | Reinsurer concentration risk | Not Rated | All Other Carriers
Ceded Credit Risk [Line Items]
Concentration risk percentage	[1]	36.10%	36.20%
Stockholders' equity | Reinsurer concentration risk | Largest Balance Due From Any One Carrier
Ceded Credit Risk [Line Items]
Concentration risk percentage		7.80%	7.80%

[1] At September 30, 2025 and December 31, 2024 over 95% of such amount were collateralized through reinsurance trusts, funds withheld arrangements, letters of credit or other