Reinsurance and Indemnification - National Flood Insurance Program (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Sep. 30, 2023
Reinsurance Retention Policy [Line Items]
Property-liability insurance premiums earned	$ 2,036	$ 1,906	$ 1,983
National Flood Insurance Program
Reinsurance Retention Policy [Line Items]
Legislation to reauthorize				$ 20,500
Reinsurance and indemnification recoverables, net	76	145
Property-liability insurance premiums earned	327	319	350
Reinsurance ceded amount	$ 102	$ 435	$ 267