Reserve for Future Policy Benefits and Contractholder Funds - Schedule of Undiscounted and Discounted Expected Gross Premiums and Expected Future Benefits and Expenses (Details) - USD ($) $ in Millions	Sep. 30, 2024	Sep. 30, 2023
Accident and health insurance
Liability for Future Policy Benefit, Activity [Line Items]
Expected future gross premiums, undiscounted	$ 5,581	$ 5,263
Expected future gross premiums, discounted	3,939	3,600
Expected future benefits and expenses, undiscounted	3,943	3,503
Expected future benefits and expenses, discounted	2,729	2,333
Traditional life
Liability for Future Policy Benefit, Activity [Line Items]
Expected future gross premiums, undiscounted	1,013	813
Expected future gross premiums, discounted	729	553
Expected future benefits and expenses, undiscounted	1,345	1,153
Expected future benefits and expenses, discounted	$ 718	$ 585