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, 2019	Sep. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	$ 3,106	$ 2,937	$ 2,290	$ 3,521	$ 2,290
Total realized/unrealized gains (losses) included in Net income	(8)	(6)		0	(5)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	23	(8)		87	(31)
Purchases and issuances	399	405		1,172	1,268
Sales and settlements	(90)	(89)		(488)	(241)
Transfer into Level 3	88	13		145	100
Transfer out of Level 3	(2)	(34)		(921)	(163)
Financial assets, Ending Balance	3,516	3,218		3,516	3,218
Fixed maturities
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	2,710	2,684	2,102	3,164	2,102
Total realized/unrealized gains (losses) included in Net income	0	0		10	(8)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	23	(8)		87	(31)
Purchases and issuances	381	324		1,134	1,157
Sales and settlements	(88)	(89)		(485)	(237)
Transfer into Level 3	54	13		89	70
Transfer out of Level 3	(2)	(17)		(921)	(146)
Financial assets, Ending Balance	3,078	2,907		3,078	2,907
U.S. government agency
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	8	8	8	9	8
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		(1)	0
Transfer into Level 3	0	0		0	0
Transfer out of Level 3	0	0		0	0
Financial assets, Ending Balance	8	8		8	8
State and municipal
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	82	61	148	59	148
Total realized/unrealized gains (losses) included in Net income	0	0		0	0
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	2	0		9	(2)
Purchases and issuances	0	0		0	0
Sales and settlements	0	(1)		(2)	(2)
Transfer into Level 3	18	0		36	0
Transfer out of Level 3	0	0		0	(84)
Financial assets, Ending Balance	102	60		102	60
Residential MBS
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	139	147	122	197	122
Total realized/unrealized gains (losses) included in Net income	1	(2)		10	(9)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	(1)	(2)		(6)	(2)
Purchases and issuances	0	0		0	0
Sales and settlements	(4)	(6)		(14)	(17)
Transfer into Level 3	22	13		24	70
Transfer out of Level 3	(1)	(5)		(55)	(19)
Financial assets, Ending Balance	156	145		156	145
Commercial MBS
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	50	56	36	56	36
Total realized/unrealized gains (losses) included in Net income	1	2		3	1
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	0		0	0
Purchases and issuances	0	(1)		0	20
Sales and settlements	0	0		(3)	0
Transfer into Level 3	4	0		4	0
Transfer out of Level 3	0	0		(5)	0
Financial assets, Ending Balance	55	57		55	57
Collateralized loan obligations
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	50	212	180	116	180
Total realized/unrealized gains (losses) included in Net income	(2)	0		(5)	(2)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	1	(2)		7	(3)
Purchases and issuances	8	0		8	35
Sales and settlements	0	0		0	0
Transfer into Level 3	0	0		13	0
Transfer out of Level 3	0	0		(82)	0
Financial assets, Ending Balance	57	210		57	210
Other asset-backed securities
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	367	792	564	731	564
Total realized/unrealized gains (losses) included in Net income	0	0		0	0
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	1	(1)		6	(3)
Purchases and issuances	49	13		141	318
Sales and settlements	(3)	(23)		(135)	(80)
Transfer into Level 3	0	0		0	0
Transfer out of Level 3	0	0		(329)	(18)
Financial assets, Ending Balance	414	781		414	781
Corporate and other
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	2,014	1,408	1,044	1,996	1,044
Total realized/unrealized gains (losses) included in Net income	0	0		2	2
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	20	(3)		71	(21)
Purchases and issuances	324	312		985	784
Sales and settlements	(81)	(59)		(330)	(138)
Transfer into Level 3	10	0		12	0
Transfer out of Level 3	(1)	(12)		(450)	(25)
Financial assets, Ending Balance	2,286	1,646		2,286	1,646
Equity securities
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	377	230	165	336	165
Total realized/unrealized gains (losses) included in Net income	(7)	(5)		(7)	9
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	0		0	0
Purchases and issuances	18	81		38	106
Sales and settlements	(2)	0		(3)	(4)
Transfer into Level 3	34	0	29	56	30
Transfer out of Level 3	0	(17)		0	(17)
Financial assets, Ending Balance	420	289		420	289
Assets of MIE
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	19	23	$ 23	21	23
Total realized/unrealized gains (losses) included in Net income	(1)	(1)		(3)	(6)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	0		0	0
Purchases and issuances	0	0		0	5
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	$ 18	$ 22		$ 18	$ 22