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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	$ 1,976	$ 1,771	$ 1,762	$ 1,636
Total realized/unrealized gains (losses) included in Net income	(13)	(8)	(18)	(28)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	15	25	29	45
Purchases and issuances	193	23	432	125
Sales and settlements	(79)	(87)	(136)	(110)
Transfer into Level 3	32	32	89	104
Transfer out of Level 3	(133)	(31)	(167)	(47)
Financial assets, Ending Balance	1,991	1,725	1,991	1,725
Fixed maturities
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	1,777	1,589	1,559	1,470
Total realized/unrealized gains (losses) included in Net income	2	(6)	4	(7)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	9	17	16	29
Purchases and issuances	183	19	408	109
Sales and settlements	(76)	(87)	(133)	(110)
Transfer into Level 3	32	32	89	89
Transfer out of Level 3	(127)	(31)	(143)	(47)
Financial assets, Ending Balance	1,800	1,533	1,800	1,533
U.S. government agency
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	8	15	8	15
Total realized/unrealized gains (losses) included in Net income	0	(8)	0	(8)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	1	0	1
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	143	92	140	89
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	0	4	3
Purchases and issuances	0	0	0	0
Sales and settlements	(1)	(1)	(1)	(1)
Transfer into Level 3	0	0	0	0
Transfer out of Level 3	0	0	0	0
Financial assets, Ending Balance	143	91	143	91
Residential MBS
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	175	213	190	224
Total realized/unrealized gains (losses) included in Net income	(3)	1	(2)	2
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	2	1	2	1
Purchases and issuances	0	0	1	0
Sales and settlements	(23)	(6)	(31)	(13)
Transfer into Level 3	13	22	20	33
Transfer out of Level 3	(11)	0	(27)	(16)
Financial assets, Ending Balance	153	231	153	231
Commercial MBS
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	29	38	25	39
Total realized/unrealized gains (losses) included in Net income	1	(1)	1	(1)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	0	0	0
Purchases and issuances	15	0	15	0
Sales and settlements	0	(1)	0	(2)
Transfer into Level 3	0	0	4	0
Transfer out of Level 3	0	0	0	0
Financial assets, Ending Balance	45	36	45	36
Asset-backed securities
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	594	501	484	470
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	3	2	(3)
Purchases and issuances	0	11	104	15
Sales and settlements	(25)	(11)	(36)	(19)
Transfer into Level 3	19	0	36	41
Transfer out of Level 3	(92)	(26)	(92)	(26)
Financial assets, Ending Balance	498	478	498	478
Corporate and other
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	828	730	712	633
Total realized/unrealized gains (losses) included in Net income	4	2	5	0
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	4	12	8	27
Purchases and issuances	168	8	288	94
Sales and settlements	(27)	(68)	(65)	(75)
Transfer into Level 3	0	10	29	15
Transfer out of Level 3	(24)	(5)	(24)	(5)
Financial assets, Ending Balance	953	689	953	689
Equity securities
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	173	158	174	140
Total realized/unrealized gains (losses) included in Net income	(10)	0	(16)	(17)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	6	8	13	16
Purchases and issuances	8	0	20	12
Sales and settlements	(3)	0	(3)	0
Transfer into Level 3	0	0	0	15
Transfer out of Level 3	(6)	0	(20)	0
Financial assets, Ending Balance	168	166	168	166
Assets of MIE
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	26	24	29	26
Total realized/unrealized gains (losses) included in Net income	(5)	(2)	(6)	(4)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	0	0	0
Purchases and issuances	2	4	4	4
Sales and settlements	0	0	0	0
Transfer into Level 3	0	0	0	0
Transfer out of Level 3	0	0	(4)	0
Financial assets, Ending Balance	$ 23	$ 26	$ 23	$ 26