Separate Account Assets and Liabilities - Separate account assets (Details) - USD ($) $ in Millions	Sep. 30, 2023	Sep. 30, 2022
Fair Value, Separate Account Investment [Line Items]
Value of investment options	$ 84,724	$ 81,302
Equity Funds | Equity Funds
Fair Value, Separate Account Investment [Line Items]
Value of investment options	51,025	46,117
Bond Funds | Bond funds
Fair Value, Separate Account Investment [Line Items]
Value of investment options	8,365	8,904
Balanced Funds | Balanced funds
Fair Value, Separate Account Investment [Line Items]
Value of investment options	23,711	24,606
Money Market Funds | Money market funds
Fair Value, Separate Account Investment [Line Items]
Value of investment options	1,623	1,675
Individual Retirement
Fair Value, Separate Account Investment [Line Items]
Value of investment options	44,613	43,385
Individual Retirement | Equity Funds | Equity Funds
Fair Value, Separate Account Investment [Line Items]
Value of investment options	23,493	21,696
Individual Retirement | Bond Funds | Bond funds
Fair Value, Separate Account Investment [Line Items]
Value of investment options	3,775	3,661
Individual Retirement | Balanced Funds | Balanced funds
Fair Value, Separate Account Investment [Line Items]
Value of investment options	16,649	17,346
Individual Retirement | Money Market Funds | Money market funds
Fair Value, Separate Account Investment [Line Items]
Value of investment options	696	682
Group Retirement
Fair Value, Separate Account Investment [Line Items]
Value of investment options	35,166	32,528
Group Retirement | Equity Funds | Equity Funds
Fair Value, Separate Account Investment [Line Items]
Value of investment options	26,206	23,226
Group Retirement | Bond Funds | Bond funds
Fair Value, Separate Account Investment [Line Items]
Value of investment options	3,262	3,910
Group Retirement | Balanced Funds | Balanced funds
Fair Value, Separate Account Investment [Line Items]
Value of investment options	5,095	4,853
Group Retirement | Money Market Funds | Money market funds
Fair Value, Separate Account Investment [Line Items]
Value of investment options	603	539
Life Insurance
Fair Value, Separate Account Investment [Line Items]
Value of investment options	848	757
Life Insurance | Equity Funds | Equity Funds
Fair Value, Separate Account Investment [Line Items]
Value of investment options	736	648
Life Insurance | Bond Funds | Bond funds
Fair Value, Separate Account Investment [Line Items]
Value of investment options	42	46
Life Insurance | Balanced Funds | Balanced funds
Fair Value, Separate Account Investment [Line Items]
Value of investment options	48	46
Life Insurance | Money Market Funds | Money market funds
Fair Value, Separate Account Investment [Line Items]
Value of investment options	22	17
Institutional Markets
Fair Value, Separate Account Investment [Line Items]
Value of investment options	4,097	4,632
Institutional Markets | Equity Funds | Equity Funds
Fair Value, Separate Account Investment [Line Items]
Value of investment options	590	547
Institutional Markets | Bond Funds | Bond funds
Fair Value, Separate Account Investment [Line Items]
Value of investment options	1,286	1,287
Institutional Markets | Balanced Funds | Balanced funds
Fair Value, Separate Account Investment [Line Items]
Value of investment options	1,919	2,361
Institutional Markets | Money Market Funds | Money market funds
Fair Value, Separate Account Investment [Line Items]
Value of investment options	$ 302	$ 437