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, 2025	Mar. 31, 2024
Accident and health insurance
Liability for Future Policy Benefit, Activity [Line Items]
Expected future gross premiums, undiscounted	$ 6,039	$ 5,580
Expected future gross premiums, discounted	4,057	3,807
Expected future benefits and expenses, undiscounted	4,325	3,754
Expected future benefits and expenses, discounted	2,864	2,509
Traditional life
Liability for Future Policy Benefit, Activity [Line Items]
Expected future gross premiums, undiscounted	1,127	1,002
Expected future gross premiums, discounted	786	687
Expected future benefits and expenses, undiscounted	1,475	1,399
Expected future benefits and expenses, discounted	$ 736	$ 672