Reserve for Property and Casualty Insurance Claims and Claims Expense - Schedule of Prior Year Reserve Re-Estimates Included in Claims and Claims Expense (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Dec. 31, 2024
Increase (decrease) in claims and claims expense
Prior years	$ (251)	$ (151)
Auto
Increase (decrease) in claims and claims expense
Prior years	(249)	(74)
Homeowners
Increase (decrease) in claims and claims expense
Prior years	(8)	(189)
Other personal lines
Increase (decrease) in claims and claims expense
Prior years	46	52
Commercial lines
Increase (decrease) in claims and claims expense
Prior years	(28)	51
Other business lines
Increase (decrease) in claims and claims expense
Prior years	(15)	5
Run-off Property-Liability
Increase (decrease) in claims and claims expense
Prior years	3	4
Catastrophe losses
Increase (decrease) in claims and claims expense
Prior years	(16)	(162)
Catastrophe losses | Auto
Increase (decrease) in claims and claims expense
Prior years	(11)	(7)
Catastrophe losses | Homeowners
Increase (decrease) in claims and claims expense
Prior years	(1)	(149)
Catastrophe losses | Other personal lines
Increase (decrease) in claims and claims expense
Prior years	(7)	(3)
Catastrophe losses | Commercial lines
Increase (decrease) in claims and claims expense
Prior years	3	(3)
Catastrophe losses | Other business lines
Increase (decrease) in claims and claims expense
Prior years	0	0
Catastrophe losses | Run-off Property-Liability
Increase (decrease) in claims and claims expense
Prior years	0	0
Non-catastrophe losses
Increase (decrease) in claims and claims expense
Prior years	(235)	11
Non-catastrophe losses | Auto
Increase (decrease) in claims and claims expense
Prior years	(238)	(67)
Non-catastrophe losses | Homeowners
Increase (decrease) in claims and claims expense
Prior years	(7)	(40)
Non-catastrophe losses | Other personal lines
Increase (decrease) in claims and claims expense
Prior years	53	55
Non-catastrophe losses | Commercial lines
Increase (decrease) in claims and claims expense
Prior years	(31)	54
Non-catastrophe losses | Other business lines
Increase (decrease) in claims and claims expense
Prior years	(15)	5
Non-catastrophe losses | Run-off Property-Liability
Increase (decrease) in claims and claims expense
Prior years	$ 3	$ 4
Nationwide Reinsurance Program | Run-off Property-Liability
Increase (decrease) in claims and claims expense
Prior years			$ 66