Separate Account Assets and Liabilities - Separate account assets (Details) - USD ($) $ in Millions	Mar. 31, 2023	Mar. 31, 2022
Individual Retirement
Fair Value, Separate Account Investment [Line Items]
Value of investment options	$ 46,296	$ 53,338
Individual Retirement | Equity Funds | Equity Funds
Fair Value, Separate Account Investment [Line Items]
Value of investment options	23,845	26,931
Individual Retirement | Bond Funds | Bond funds
Fair Value, Separate Account Investment [Line Items]
Value of investment options	3,915	4,401
Individual Retirement | Balanced Funds | Balanced funds
Fair Value, Separate Account Investment [Line Items]
Value of investment options	17,818	21,402
Individual Retirement | Money Market Funds | Money market funds
Fair Value, Separate Account Investment [Line Items]
Value of investment options	718	604
Group Retirement
Fair Value, Separate Account Investment [Line Items]
Value of investment options	35,792	41,342
Group Retirement | Equity Funds | Equity Funds
Fair Value, Separate Account Investment [Line Items]
Value of investment options	26,433	30,844
Group Retirement | Bond Funds | Bond funds
Fair Value, Separate Account Investment [Line Items]
Value of investment options	3,571	4,176
Group Retirement | Balanced Funds | Balanced funds
Fair Value, Separate Account Investment [Line Items]
Value of investment options	5,254	5,866
Group Retirement | Money Market Funds | Money market funds
Fair Value, Separate Account Investment [Line Items]
Value of investment options	$ 534	$ 456