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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	$ 1,991	$ 1,725	$ 1,762	$ 1,636
Total realized/unrealized gains (losses) included in Net income	(15)	0	(33)	(28)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	(2)	7	27	52
Purchases and issuances	189	47	621	172
Sales and settlements	(102)	(51)	(238)	(161)
Transfer into Level 3	188	0	277	104
Transfer out of Level 3	(139)	(11)	(306)	(58)
Financial assets, Ending Balance	2,110	1,717	2,110	1,717
Fixed maturities
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	1,800	1,533	1,559	1,470
Total realized/unrealized gains (losses) included in Net income	(8)	(3)	(4)	(10)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	2	2	18	31
Purchases and issuances	185	37	593	146
Sales and settlements	(102)	(30)	(235)	(140)
Transfer into Level 3	188	0	277	89
Transfer out of Level 3	(139)	(11)	(282)	(58)
Financial assets, Ending Balance	1,926	1,528	1,926	1,528
U.S. government agency
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	8	8	8	15
Total realized/unrealized gains (losses) included in Net income	0	0	0	(8)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	0	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	91	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)	0	1	4	4
Purchases and issuances	0	0	0	0
Sales and settlements	(1)	(1)	(2)	(2)
Transfer into Level 3	10	0	10	0
Transfer out of Level 3	0	0	0	0
Financial assets, Ending Balance	152	91	152	91
Residential MBS
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	153	231	190	224
Total realized/unrealized gains (losses) included in Net income	2	(2)	0	0
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	1	0	3	1
Purchases and issuances	0	0	1	0
Sales and settlements	(6)	(8)	(37)	(21)
Transfer into Level 3	15	0	35	33
Transfer out of Level 3	(21)	(2)	(48)	(18)
Financial assets, Ending Balance	144	219	144	219
Commercial MBS
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	45	36	25	39
Total realized/unrealized gains (losses) included in Net income	1	0	2	(1)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	0	0	0
Purchases and issuances	0	0	15	0
Sales and settlements	(10)	(2)	(10)	(4)
Transfer into Level 3	0	0	4	0
Transfer out of Level 3	0	0	0	0
Financial assets, Ending Balance	36	34	36	34
Asset-backed securities
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	498	478	484	470
Total realized/unrealized gains (losses) included in Net income	(2)	(1)	(2)	(1)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	1	4	3	1
Purchases and issuances	13	0	117	15
Sales and settlements	(26)	(5)	(62)	(24)
Transfer into Level 3	163	0	199	41
Transfer out of Level 3	(111)	(9)	(203)	(35)
Financial assets, Ending Balance	536	467	536	467
Corporate and other
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	953	689	712	633
Total realized/unrealized gains (losses) included in Net income	(9)	0	(4)	0
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	(3)	8	24
Purchases and issuances	172	37	460	131
Sales and settlements	(59)	(14)	(124)	(89)
Transfer into Level 3	0	0	29	15
Transfer out of Level 3	(7)	0	(31)	(5)
Financial assets, Ending Balance	1,050	709	1,050	709
Equity securities
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	168	166	174	140
Total realized/unrealized gains (losses) included in Net income	(3)	5	(19)	(12)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	(4)	5	9	21
Purchases and issuances	2	10	22	22
Sales and settlements	0	(21)	(3)	(21)
Transfer into Level 3	0	0	0	15
Transfer out of Level 3	0	0	(20)	0
Financial assets, Ending Balance	163	165	163	165
Assets of MIE
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	23	26	29	26
Total realized/unrealized gains (losses) included in Net income	(4)	(2)	(10)	(6)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	0	0	0
Purchases and issuances	2	0	6	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	$ 21	$ 24	$ 21	$ 24