Reserve for Future Policy Benefits and Contractholder Funds - Schedule of Account Values by Comparison of Current Crediting Rate to Guaranteed Minimum Crediting Rate (Details) $ in Millions	Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)
Policyholder Account Balance [Line Items]
Account values with crediting rates	$ 884	$ 893
Less than 3.00%
Policyholder Account Balance [Line Items]
Account values with crediting rates	$ 0	$ 0
Less than 3.00% | Maximum
Policyholder Account Balance [Line Items]
Range of guaranteed minimum crediting rates	3.00%	3.00%
3.00% - 3.49%
Policyholder Account Balance [Line Items]
Account values with crediting rates	$ 27	$ 13
3.00% - 3.49% | Minimum
Policyholder Account Balance [Line Items]
Range of guaranteed minimum crediting rates	3.00%	3.00%
3.00% - 3.49% | Maximum
Policyholder Account Balance [Line Items]
Range of guaranteed minimum crediting rates	3.49%	3.49%
3.50% - 3.99%
Policyholder Account Balance [Line Items]
Account values with crediting rates	$ 11	$ 12
3.50% - 3.99% | Minimum
Policyholder Account Balance [Line Items]
Range of guaranteed minimum crediting rates	3.50%	3.50%
3.50% - 3.99% | Maximum
Policyholder Account Balance [Line Items]
Range of guaranteed minimum crediting rates	3.99%	3.99%
4.00% - 4.49%
Policyholder Account Balance [Line Items]
Account values with crediting rates	$ 437	$ 438
4.00% - 4.49% | Minimum
Policyholder Account Balance [Line Items]
Range of guaranteed minimum crediting rates	4.00%	4.00%
4.00% - 4.49% | Maximum
Policyholder Account Balance [Line Items]
Range of guaranteed minimum crediting rates	4.49%	4.49%
4.50% - 4.99%
Policyholder Account Balance [Line Items]
Account values with crediting rates	$ 263	$ 269
4.50% - 4.99% | Minimum
Policyholder Account Balance [Line Items]
Range of guaranteed minimum crediting rates	4.50%	4.50%
4.50% - 4.99% | Maximum
Policyholder Account Balance [Line Items]
Range of guaranteed minimum crediting rates	4.99%	4.99%
5.00% or greater
Policyholder Account Balance [Line Items]
Account values with crediting rates	$ 68	$ 70
5.00% or greater | Minimum
Policyholder Account Balance [Line Items]
Range of guaranteed minimum crediting rates	5.00%	5.00%
Non-account balances
Policyholder Account Balance [Line Items]
Account values with crediting rates	$ 78	$ 91
At guaranteed minimum
Policyholder Account Balance [Line Items]
Account values with crediting rates	779	789
At guaranteed minimum | Less than 3.00%
Policyholder Account Balance [Line Items]
Account values with crediting rates	0	0
At guaranteed minimum | 3.00% - 3.49%
Policyholder Account Balance [Line Items]
Account values with crediting rates	0	0
At guaranteed minimum | 3.50% - 3.99%
Policyholder Account Balance [Line Items]
Account values with crediting rates	11	12
At guaranteed minimum | 4.00% - 4.49%
Policyholder Account Balance [Line Items]
Account values with crediting rates	437	438
At guaranteed minimum | 4.50% - 4.99%
Policyholder Account Balance [Line Items]
Account values with crediting rates	263	269
At guaranteed minimum | 5.00% or greater
Policyholder Account Balance [Line Items]
Account values with crediting rates	68	70
At guaranteed minimum | Non-account balances
Policyholder Account Balance [Line Items]
Account values with crediting rates
1-50 basis points above
Policyholder Account Balance [Line Items]
Account values with crediting rates	$ 27	13
1-50 basis points above | Minimum
Policyholder Account Balance [Line Items]
Policyholder account balance above guaranteed minimum crediting rate	0.0001
1-50 basis points above | Maximum
Policyholder Account Balance [Line Items]
Policyholder account balance above guaranteed minimum crediting rate	0.0050
1-50 basis points above | Less than 3.00%
Policyholder Account Balance [Line Items]
Account values with crediting rates	$ 0	0
1-50 basis points above | 3.00% - 3.49%
Policyholder Account Balance [Line Items]
Account values with crediting rates	27	13
1-50 basis points above | 3.50% - 3.99%
Policyholder Account Balance [Line Items]
Account values with crediting rates	0	0
1-50 basis points above | 4.00% - 4.49%
Policyholder Account Balance [Line Items]
Account values with crediting rates	0	0
1-50 basis points above | 4.50% - 4.99%
Policyholder Account Balance [Line Items]
Account values with crediting rates	0	0
1-50 basis points above | 5.00% or greater
Policyholder Account Balance [Line Items]
Account values with crediting rates	0	0
1-50 basis points above | Non-account balances
Policyholder Account Balance [Line Items]
Account values with crediting rates