Fair Value Measurements - Changes in balances of Level 3 financial assets carried at fair value (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Jun. 30, 2023	Dec. 31, 2022	Jun. 30, 2022	Dec. 31, 2021
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	$ 1,160	$ 983	$ 1,102	$ 926
Total realized/unrealized gains (losses) included in Net income	(12)	(4)	(8)	18
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	(3)	(20)	11	(53)
Purchases and issuances	62	74	191	243
Sales and settlements	(20)	(29)	(92)	(88)
Transfer into Level 3	5	4	42	43
Transfer out of Level 3	(38)	(6)	(92)	(87)
Financial assets, Ending Balance	1,154	1,002	1,154	1,002
Contingent consideration - acquisitions, Total Level 3 liabilities	(2)	(25)	(2)	(25)	$ (25)	$ (25)	$ (23)	$ (23)
Contingent consideration — acquisitions
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Contingent consideration - acquisitions, Total Level 3 liabilities	(2)	(25)	(2)	(25)	$ (25)	$ (25)	$ (23)	$ (23)
Fixed maturities:
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	701	593	664	600
Total realized/unrealized gains (losses) included in Net income	(15)	(1)	(16)	1
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	(3)	(20)	11	(53)
Purchases and issuances	39	50	105	167
Sales and settlements	(16)	(14)	(63)	(68)
Transfer into Level 3	5	4	42	40
Transfer out of Level 3	(21)	(5)	(53)	(80)
Financial assets, Ending Balance	690	607	690	607
U.S. government agency
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	0	0	0	0
Total realized/unrealized gains (losses) included in Net income	0	0	0	0
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	0	0	0
Purchases and issuances	0	0	0	0
Sales and settlements	0	0	0	0
Transfer into Level 3	0	0	0	0
Transfer out of Level 3	0	0	0	0
Financial assets, Ending Balance	0	0	0	0
State and municipal
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	5	1	5	41
Total realized/unrealized gains (losses) included in Net income	0	0	0	0
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	0	0	(3)
Purchases and issuances	0	0	0	0
Sales and settlements	(1)	0	(1)	(1)
Transfer into Level 3	0	0	0	0
Transfer out of Level 3	(2)	0	(2)	(36)
Financial assets, Ending Balance	2	1	2	1
Residential MBS
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	5	8	9	14
Total realized/unrealized gains (losses) included in Net income	0	0	0	0
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	0	0	0
Purchases and issuances	0	0	0	0
Sales and settlements	0	0	(3)	(1)
Transfer into Level 3	0	4	4	4
Transfer out of Level 3	0	(4)	(5)	(9)
Financial assets, Ending Balance	5	8	5	8
Commercial MBS
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	0	0	0	0
Total realized/unrealized gains (losses) included in Net income	0	0	0	0
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	0	0	0
Purchases and issuances	0	0	0	0
Sales and settlements	0	0	0	0
Transfer into Level 3	0	0	0	0
Transfer out of Level 3	0	0	0	0
Financial assets, Ending Balance	0	0	0	0
Collateralized loan obligations
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	1	2	2	0
Total realized/unrealized gains (losses) included in Net income	(1)	0	(1)	0
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	1	0	1	0
Purchases and issuances	0	0	0	0
Sales and settlements	0	0	0	0
Transfer into Level 3	0	0	0	2
Transfer out of Level 3	0	0	(1)	0
Financial assets, Ending Balance	1	2	1	2
Other asset-backed securities
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	321	313	329	278
Total realized/unrealized gains (losses) included in Net income	0	0	(2)	2
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	(4)	(8)	0	(24)
Purchases and issuances	28	5	38	62
Sales and settlements	(3)	(9)	(34)	(51)
Transfer into Level 3	0	0	31	34
Transfer out of Level 3	(12)	0	(32)	0
Financial assets, Ending Balance	330	301	330	301
Corporate and other
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	369	269	319	267
Total realized/unrealized gains (losses) included in Net income	(14)	(1)	(13)	(1)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	(12)	10	(26)
Purchases and issuances	11	45	67	105
Sales and settlements	(12)	(5)	(25)	(15)
Transfer into Level 3	5	0	7	0
Transfer out of Level 3	(7)	(1)	(13)	(35)
Financial assets, Ending Balance	352	295	352	295
Equity securities
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	448	378	427	313
Total realized/unrealized gains (losses) included in Net income	4	(2)	11	20
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	0	0	0
Purchases and issuances	23	24	84	75
Sales and settlements	(4)	(15)	(29)	(20)
Transfer into Level 3	0	0	0	3
Transfer out of Level 3	(17)	(1)	(39)	(7)
Financial assets, Ending Balance	454	384	454	384
Assets of MIE
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	11	12	11	13
Total realized/unrealized gains (losses) included in Net income	(1)	(1)	(3)	(3)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	0	0	0
Purchases and issuances	0	0	2	1
Sales and settlements	0	0	0	0
Transfer into Level 3	0	0	0	0
Transfer out of Level 3	0	0	0	0
Financial assets, Ending Balance	$ 10	$ 11	$ 10	$ 11