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	Mar. 31, 2024	Mar. 31, 2023
Accident and health insurance
Liability for Future Policy Benefit, Activity [Line Items]
Expected future gross premiums, undiscounted	$ 5,580	$ 5,068
Expected future gross premiums, discounted	3,807	3,671
Expected future benefits and expenses, undiscounted	3,754	3,351
Expected future benefits and expenses, discounted	2,509	2,335
Traditional life
Liability for Future Policy Benefit, Activity [Line Items]
Expected future gross premiums, undiscounted	1,002	721
Expected future gross premiums, discounted	687	500
Expected future benefits and expenses, undiscounted	1,399	1,008
Expected future benefits and expenses, discounted	$ 672	$ 547