Reinsurance - Reinsurance Recoverables (Details) - USD ($) $ in Millions	Sep. 30, 2024	Sep. 30, 2023
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	$ 3,961
Allowance for uncollectible reinsurance, including assets held for sale	(30)
Market risk benefits	1,008	$ 993
Total reinsurance recoverables, including assets held for sale	4,939
Held-for-Sale | Medicare Advantage and related Cigna Healthcare businesses
Ceded Credit Risk [Line Items]
Reinsurance recoverable classified as assets of businesses held for sale	194
Other Current Assets
Ceded Credit Risk [Line Items]
Reinsurance recoverables	188
Berkshire Hathway Life Insurance Company Of Nebraska and Other Recoverables
Ceded Credit Risk [Line Items]
Market risk benefits	1,008
Fair value of collateral contractually required to meet or exceed carrying value of recoverable
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	618
Collateral provisions exist that may mitigate risk of credit loss
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	2,902
No collateral
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	441
Ongoing Operations
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	461
Ongoing Operations | A- equivalent and higher current ratings
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	242
Ongoing Operations | BBB- to BBB+ equivalent current credit ratings
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	62
Ongoing Operations | Not rated
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	157
Ongoing Operations | Fair value of collateral contractually required to meet or exceed carrying value of recoverable
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	143
Ongoing Operations | Fair value of collateral contractually required to meet or exceed carrying value of recoverable | A- equivalent and higher current ratings
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	0
Ongoing Operations | Fair value of collateral contractually required to meet or exceed carrying value of recoverable | BBB- to BBB+ equivalent current credit ratings
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	0
Ongoing Operations | Fair value of collateral contractually required to meet or exceed carrying value of recoverable | Not rated
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	143
Ongoing Operations | Collateral provisions exist that may mitigate risk of credit loss
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	18
Ongoing Operations | Collateral provisions exist that may mitigate risk of credit loss | A- equivalent and higher current ratings
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	7
Ongoing Operations | Collateral provisions exist that may mitigate risk of credit loss | BBB- to BBB+ equivalent current credit ratings
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	0
Ongoing Operations | Collateral provisions exist that may mitigate risk of credit loss | Not rated
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	11
Ongoing Operations | No collateral
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	300
Ongoing Operations | No collateral | A- equivalent and higher current ratings
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	235
Ongoing Operations | No collateral | BBB- to BBB+ equivalent current credit ratings
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	62
Ongoing Operations | No collateral | Not rated
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	3
Acquisition, disposition or run-off activities
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	3,500
Acquisition, disposition or run-off activities | BBB+ equivalent and higher current ratings | Lincoln National Life and Lincoln Life & Annuity of New York
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	2,548
Acquisition, disposition or run-off activities | BBB+ equivalent and higher current ratings | Empower Annuity Insurance Company
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	123
Acquisition, disposition or run-off activities | BBB+ equivalent and higher current ratings | Prudential Insurance Company of America
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	320
Acquisition, disposition or run-off activities | BBB+ equivalent and higher current ratings | Life Insurance Company of North America
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	311
Acquisition, disposition or run-off activities | BBB+ equivalent and higher current ratings | Other
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	188
Acquisition, disposition or run-off activities | Not rated
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	10
Acquisition, disposition or run-off activities | Fair value of collateral contractually required to meet or exceed carrying value of recoverable
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	475
Acquisition, disposition or run-off activities | Fair value of collateral contractually required to meet or exceed carrying value of recoverable | BBB+ equivalent and higher current ratings | Lincoln National Life and Lincoln Life & Annuity of New York
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	0
Acquisition, disposition or run-off activities | Fair value of collateral contractually required to meet or exceed carrying value of recoverable | BBB+ equivalent and higher current ratings | Empower Annuity Insurance Company
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	0
Acquisition, disposition or run-off activities | Fair value of collateral contractually required to meet or exceed carrying value of recoverable | BBB+ equivalent and higher current ratings | Prudential Insurance Company of America
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	320
Acquisition, disposition or run-off activities | Fair value of collateral contractually required to meet or exceed carrying value of recoverable | BBB+ equivalent and higher current ratings | Life Insurance Company of North America
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	0
Acquisition, disposition or run-off activities | Fair value of collateral contractually required to meet or exceed carrying value of recoverable | BBB+ equivalent and higher current ratings | Other
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	155
Acquisition, disposition or run-off activities | Fair value of collateral contractually required to meet or exceed carrying value of recoverable | Not rated
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	0
Acquisition, disposition or run-off activities | Collateral provisions exist that may mitigate risk of credit loss
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	2,884
Acquisition, disposition or run-off activities | Collateral provisions exist that may mitigate risk of credit loss | BBB+ equivalent and higher current ratings | Lincoln National Life and Lincoln Life & Annuity of New York
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	2,548
Acquisition, disposition or run-off activities | Collateral provisions exist that may mitigate risk of credit loss | BBB+ equivalent and higher current ratings | Empower Annuity Insurance Company
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	0
Acquisition, disposition or run-off activities | Collateral provisions exist that may mitigate risk of credit loss | BBB+ equivalent and higher current ratings | Prudential Insurance Company of America
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	0
Acquisition, disposition or run-off activities | Collateral provisions exist that may mitigate risk of credit loss | BBB+ equivalent and higher current ratings | Life Insurance Company of North America
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	311
Acquisition, disposition or run-off activities | Collateral provisions exist that may mitigate risk of credit loss | BBB+ equivalent and higher current ratings | Other
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	19
Acquisition, disposition or run-off activities | Collateral provisions exist that may mitigate risk of credit loss | Not rated
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	6
Acquisition, disposition or run-off activities | No collateral
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	141
Acquisition, disposition or run-off activities | No collateral | BBB+ equivalent and higher current ratings | Lincoln National Life and Lincoln Life & Annuity of New York
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	0
Acquisition, disposition or run-off activities | No collateral | BBB+ equivalent and higher current ratings | Empower Annuity Insurance Company
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	123
Acquisition, disposition or run-off activities | No collateral | BBB+ equivalent and higher current ratings | Prudential Insurance Company of America
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	0
Acquisition, disposition or run-off activities | No collateral | BBB+ equivalent and higher current ratings | Life Insurance Company of North America
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	0
Acquisition, disposition or run-off activities | No collateral | BBB+ equivalent and higher current ratings | Other
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	14
Acquisition, disposition or run-off activities | No collateral | Not rated
Ceded Credit Risk [Line Items]
Reinsurance recoverables before market risk benefits, including assets held for sale	$ 4