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, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	$ 3,685	$ 2,600	$ 2,290	$ 3,521	$ 2,290
Total realized/unrealized gains (losses) included in Net income	4	14		8	1
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	25	(11)		64	(23)
Purchases and issuances	265	397		773	863
Sales and settlements	(187)	(73)		(398)	(152)
Transfer into Level 3	41	51		57	87
Transfer out of Level 3	(727)	(41)		(919)	(129)
Financial assets, Ending Balance	3,106	2,937	2,600	3,106	2,937
Fixed maturities
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	3,311	2,382	2,102	3,164	2,102
Total realized/unrealized gains (losses) included in Net income	6	(2)		10	(8)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	25	(11)		64	(23)
Purchases and issuances	246	379		753	833
Sales and settlements	(186)	(73)		(397)	(148)
Transfer into Level 3	35	50		35	57
Transfer out of Level 3	(727)	(41)		(919)	(129)
Financial assets, Ending Balance	2,710	2,684	2,382	2,710	2,684
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	8
State and municipal
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	63	62	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	(1)		7	(2)
Purchases and issuances	0	0		0	0
Sales and settlements	(1)	0		(2)	(1)
Transfer into Level 3	18	0		18	0
Transfer out of Level 3	0	0		0	(84)
Financial assets, Ending Balance	82	61	62	82	61
Residential MBS
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	169	115	122	197	122
Total realized/unrealized gains (losses) included in Net income	4	(3)		9	(7)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	0		(5)	0
Purchases and issuances	0	0		0	0
Sales and settlements	(4)	(5)		(10)	(11)
Transfer into Level 3	2	50		2	57
Transfer out of Level 3	(32)	(10)		(54)	(14)
Financial assets, Ending Balance	139	147	115	139	147
Commercial MBS
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	55	47	36	56	36
Total realized/unrealized gains (losses) included in Net income	2	0		2	(1)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	0		0	0
Purchases and issuances	0	9		0	21
Sales and settlements	(2)	0		(3)	0
Transfer into Level 3	0	0		0	0
Transfer out of Level 3	(5)	0		(5)	0
Financial assets, Ending Balance	50	56	47	50	56
Collateralized loan obligations
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	37	181	180	116	180
Total realized/unrealized gains (losses) included in Net income	0	0		(3)	(2)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	(4)		6	(1)
Purchases and issuances	0	35		0	35
Sales and settlements	0	0		0	0
Transfer into Level 3	13	0		13	0
Transfer out of Level 3	0	0		(82)	0
Financial assets, Ending Balance	50	212	181	50	212
Other asset-backed securities
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	633	731	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)	3	(2)		5	(2)
Purchases and issuances	17	101		92	305
Sales and settlements	(18)	(20)		(132)	(57)
Transfer into Level 3	0	0		0	0
Transfer out of Level 3	(268)	(18)		(329)	(18)
Financial assets, Ending Balance	367	792	731	367	792
Corporate and other
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	2,346	1,238	1,044	1,996	1,044
Total realized/unrealized gains (losses) included in Net income	0	1		2	2
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	20	(4)		51	(18)
Purchases and issuances	229	234		661	472
Sales and settlements	(161)	(48)		(249)	(79)
Transfer into Level 3	2	0		2	0
Transfer out of Level 3	(422)	(13)		(449)	(13)
Financial assets, Ending Balance	2,014	1,408	1,238	2,014	1,408
Equity securities
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	354	194	165	336	165
Total realized/unrealized gains (losses) included in Net income	(1)	19		0	14
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	0		0	0
Purchases and issuances	19	16		20	25
Sales and settlements	(1)	0		(1)	(4)
Transfer into Level 3	6	1	29	22	30
Transfer out of Level 3	0	0		0	0
Financial assets, Ending Balance	377	230	194	377	230
Assets of MIE
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	20	24	23	21	23
Total realized/unrealized gains (losses) included in Net income	(1)	(3)		(2)	(5)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	0		0	0
Purchases and issuances	0	2		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	$ 19	$ 23	$ 24	$ 19	$ 23