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, 2018	Mar. 31, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	$ 2,937	$ 2,290	$ 1,991	$ 2,290	$ 1,762
Total realized/unrealized gains (losses) included in Net income	(6)		(15)	(5)	(33)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	(8)		(2)	(31)	27
Purchases and issuances	405		189	1,268	621
Sales and settlements	(89)		(102)	(241)	(238)
Transfer into Level 3	13		188	100	277
Transfer out of Level 3	(34)		(139)	(163)	(306)
Financial assets, Ending Balance	3,218		2,110	3,218	2,110
Fixed maturities
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	2,684	2,102	1,800	2,102	1,559
Total realized/unrealized gains (losses) included in Net income	0		(8)	(8)	(4)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	(8)		2	(31)	18
Purchases and issuances	324		185	1,157	593
Sales and settlements	(89)		(102)	(237)	(235)
Transfer into Level 3	13		188	70	277
Transfer out of Level 3	(17)		(139)	(146)	(282)
Financial assets, Ending Balance	2,907		1,926	2,907	1,926
U.S. government agency
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	8	8	8	8	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	0	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	61	148	143	148	140
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	(2)	4
Purchases and issuances	0		0	0	0
Sales and settlements	(1)		(1)	(2)	(2)
Transfer into Level 3	0		10	0	10
Transfer out of Level 3	0		0	(84)	0
Financial assets, Ending Balance	60		152	60	152
Residential MBS
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	147	122	153	122	190
Total realized/unrealized gains (losses) included in Net income	(2)		2	(9)	0
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	(2)		1	(2)	3
Purchases and issuances	0		0	0	1
Sales and settlements	(6)		(6)	(17)	(37)
Transfer into Level 3	13		15	70	35
Transfer out of Level 3	(5)		(21)	(19)	(48)
Financial assets, Ending Balance	145		144	145	144
Commercial MBS
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	56	36	45	36	25
Total realized/unrealized gains (losses) included in Net income	2		1	1	2
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0		0	0	0
Purchases and issuances	(1)		0	20	15
Sales and settlements	0		(10)	0	(10)
Transfer into Level 3	0		0	0	4
Transfer out of Level 3	0		0	0	0
Financial assets, Ending Balance	57		36	57	36
Asset-backed securities
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	1,004	744	498	744	484
Total realized/unrealized gains (losses) included in Net income	0		(2)	(2)	(2)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	(3)		1	(6)	3
Purchases and issuances	13		13	353	117
Sales and settlements	(23)		(26)	(80)	(62)
Transfer into Level 3	0		163	0	199
Transfer out of Level 3	0		(111)	(18)	(203)
Financial assets, Ending Balance	991		536	991	536
Corporate and other
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	1,408	1,044	953	1,044	712
Total realized/unrealized gains (losses) included in Net income	0		(9)	2	(4)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	(3)		0	(21)	8
Purchases and issuances	312		172	784	460
Sales and settlements	(59)		(59)	(138)	(124)
Transfer into Level 3	0		0	0	29
Transfer out of Level 3	(12)		(7)	(25)	(31)
Financial assets, Ending Balance	1,646		1,050	1,646	1,050
Equity securities
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	230	165	168	165	174
Total realized/unrealized gains (losses) included in Net income	(5)		(3)	9	(19)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0		(4)	0	9
Purchases and issuances	81		2	106	22
Sales and settlements	0		0	(4)	(3)
Transfer into Level 3	0	29	0	30	0
Transfer out of Level 3	(17)		0	(17)	(20)
Financial assets, Ending Balance	289		163	289	163
Assets of MIE
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	23	$ 23	23	23	29
Total realized/unrealized gains (losses) included in Net income	(1)		(4)	(6)	(10)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0		0	0	0
Purchases and issuances	0		2	5	6
Sales and settlements	0		0	0	0
Transfer into Level 3	0		0	0	0
Transfer out of Level 3	0		0	0	(4)
Financial assets, Ending Balance	$ 22		$ 21	$ 22	$ 21