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, 2023	Sep. 30, 2022
Accident and health insurance
Liability for Future Policy Benefit, Activity [Line Items]
Expected future gross premiums, undiscounted	$ 5,263	$ 4,963
Expected future gross premiums, discounted	3,600	3,533
Expected future benefits and expenses, undiscounted	3,503	3,253
Expected future benefits and expenses, discounted	2,333	2,218
Traditional life
Liability for Future Policy Benefit, Activity [Line Items]
Expected future gross premiums, undiscounted	813	624
Expected future gross premiums, discounted	553	422
Expected future benefits and expenses, undiscounted	1,153	932
Expected future benefits and expenses, discounted	$ 585	$ 496