Long-Duration Insurance Contracts - Actual and Expected Experience of Mortality Rate and Lapse Rate (Details)	Sep. 30, 2023	Sep. 30, 2022
Whole Life
Liability for Future Policy Benefit, by Product Segment [Line Items]
Mortality, actual experience	0.70%	0.80%
Mortality, expected experience	0.70%	0.70%
Lapse rate, actual experience	3.60%	3.10%
Lapse rate, expected experience	4.70%	5.80%
Term Life | Minimum
Liability for Future Policy Benefit, by Product Segment [Line Items]
Mortality, actual experience	0.10%	0.10%
Mortality, expected experience	0.10%	0.10%
Lapse rate, actual experience	5.30%	5.60%
Lapse rate, expected experience	5.70%	6.60%
Term Life | Maximum
Liability for Future Policy Benefit, by Product Segment [Line Items]
Mortality, actual experience	0.60%	0.30%
Mortality, expected experience	1.70%	0.80%
Lapse rate, actual experience	7.30%	63.00%
Lapse rate, expected experience	32.20%	7.30%
Experience Life
Liability for Future Policy Benefit, by Product Segment [Line Items]
Mortality, actual experience	1.70%	1.60%
Mortality, expected experience	1.60%	1.40%
Lapse rate, actual experience	3.20%	3.40%
Lapse rate, expected experience	3.10%	3.20%
Limited Pay Whole Life
Liability for Future Policy Benefit, by Product Segment [Line Items]
Mortality, actual experience	0.10%	0.20%
Mortality, expected experience	0.30%	0.20%
Lapse rate, actual experience	4.30%	4.00%
Lapse rate, expected experience	5.30%	7.50%