Fair Value Measurements - Changes in balances of Level 3 financial assets carried at fair value (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022	Mar. 31, 2023	Dec. 31, 2022	Mar. 31, 2022	Dec. 31, 2021
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	$ 1,128	$ 998	$ 1,102	$ 926
Total realized/unrealized gains (losses) included in Net income	3	(1)	4	22
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	8	(12)	14	(33)
Purchases and issuances	45	64	129	169
Sales and settlements	(31)	(32)	(72)	(59)
Transfer into Level 3	33	2	37	39
Transfer out of Level 3	(26)	(36)	(54)	(81)
Financial assets, Ending Balance	1,160	983	1,160	983
Contingent consideration - acquisitions, Total Level 3 liabilities	(25)	(23)	(25)	(23)	$ (25)	$ (25)	$ (23)	$ (23)
Contingent consideration — acquisitions
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Contingent consideration - acquisitions, Total Level 3 liabilities	(25)	(23)	(25)	(23)	$ (25)	$ (25)	$ (23)	$ (23)
Fixed maturities:
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	705	625	664	600
Total realized/unrealized gains (losses) included in Net income	(6)	0	(1)	2
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	8	(12)	14	(33)
Purchases and issuances	15	42	66	117
Sales and settlements	(28)	(30)	(47)	(54)
Transfer into Level 3	33	2	37	36
Transfer out of Level 3	(26)	(34)	(32)	(75)
Financial assets, Ending Balance	701	593	701	593
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	33	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	(1)	0	(3)
Purchases and issuances	0	0	0	0
Sales and settlements	0	0	0	(1)
Transfer into Level 3	0	0	0	0
Transfer out of Level 3	0	(31)	0	(36)
Financial assets, Ending Balance	5	1	5	1
Residential MBS
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	5	11	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	0	4	0
Transfer out of Level 3	0	(3)	(5)	(5)
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	0	2	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	2	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	335	337	329	278
Total realized/unrealized gains (losses) included in Net income	(2)	0	(2)	2
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	(7)	4	(16)
Purchases and issuances	3	10	10	57
Sales and settlements	(26)	(27)	(31)	(42)
Transfer into Level 3	31	0	31	34
Transfer out of Level 3	(20)	0	(20)	0
Financial assets, Ending Balance	321	313	321	313
Corporate and other
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	359	244	319	267
Total realized/unrealized gains (losses) included in Net income	(4)	0	1	0
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	8	(4)	10	(14)
Purchases and issuances	12	32	56	60
Sales and settlements	(2)	(3)	(13)	(10)
Transfer into Level 3	2	0	2	0
Transfer out of Level 3	(6)	0	(6)	(34)
Financial assets, Ending Balance	369	269	369	269
Equity securities
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	411	361	427	313
Total realized/unrealized gains (losses) included in Net income	10	0	7	22
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	0	0	0
Purchases and issuances	30	21	61	51
Sales and settlements	(3)	(2)	(25)	(5)
Transfer into Level 3	0	0	0	3
Transfer out of Level 3	0	(2)	(22)	(6)
Financial assets, Ending Balance	448	378	448	378
Contingent consideration - acquisitions, Total Level 3 liabilities		378		378
Assets of MIE
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	12	12	11	13
Total realized/unrealized gains (losses) included in Net income	(1)	(1)	(2)	(2)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	0	0	0
Purchases and issuances	0	1	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	$ 11	$ 12	$ 11	$ 12