Market Risk Benefits - Schedule of market risk benefits (Details) - USD ($) $ in Millions	Mar. 31, 2024	Mar. 31, 2023
Market Risk Benefit [Line Items]
Asset	$ 1,172	$ 830
Liability	5,167	5,144
Net	3,995	4,314
Individual Retirement
Market Risk Benefit [Line Items]
Asset	968	685
Liability	4,714	4,680
Net	3,746	3,995
Group Retirement
Market Risk Benefit [Line Items]
Asset	204	145
Liability	453	464
Net	$ 249	$ 319