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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	$ 1,725	$ 1,519	$ 1,636	$ 1,355
Impact of accounting change				0
Total realized/unrealized gains (losses) included in Net income	0	(6)	(28)	(20)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	7	4	52	(7)
Purchases and issuances	47	150	172	423
Sales and settlements	(51)	(26)	(161)	(114)
Transfer into Level 3	0	61	104	165
Transfer out of Level 3	(11)	(11)	(58)	(111)
Financial assets, Ending Balance	1,717	1,691	1,717	1,691
Fixed maturities
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	1,533	1,372	1,470	1,231
Impact of accounting change				0
Total realized/unrealized gains (losses) included in Net income	(3)	(3)	(10)	(7)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	2	6	31	(6)
Purchases and issuances	37	143	146	367
Sales and settlements	(30)	(26)	(140)	(114)
Transfer into Level 3	0	61	89	165
Transfer out of Level 3	(11)	(11)	(58)	(94)
Financial assets, Ending Balance	1,528	1,542	1,528	1,542
U.S. government agency
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	8	15	15	15
Impact of accounting change				0
Total realized/unrealized gains (losses) included in Net income	0	0	(8)	0
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	0	1	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	15	8	15
State and municipal
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	91	84	89	100
Impact of accounting change				0
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	4	(1)
Purchases and issuances	0	9	0	34
Sales and settlements	(1)	(1)	(2)	(1)
Transfer into Level 3	0	0	0	0
Transfer out of Level 3	0	0	0	(39)
Financial assets, Ending Balance	91	93	91	93
Residential MBS
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	231	296	224	300
Impact of accounting change				0
Total realized/unrealized gains (losses) included in Net income	(2)	(3)	0	(5)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	(1)	1	0
Purchases and issuances	0	0	0	0
Sales and settlements	(8)	(8)	(21)	(24)
Transfer into Level 3	0	10	33	67
Transfer out of Level 3	(2)	0	(18)	(44)
Financial assets, Ending Balance	219	294	219	294
Commercial MBS
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	36	48	39	44
Impact of accounting change				0
Total realized/unrealized gains (losses) included in Net income	0	0	(1)	0
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	(2)	(1)	(4)	(1)
Transfer into Level 3	0	0	0	4
Transfer out of Level 3	0	(1)	0	(1)
Financial assets, Ending Balance	34	45	34	45
Asset-backed securities
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	478	332	470	226
Impact of accounting change				0
Total realized/unrealized gains (losses) included in Net income	(1)	0	(1)	1
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	4	0	1	0
Purchases and issuances	0	110	15	230
Sales and settlements	(5)	(3)	(24)	(51)
Transfer into Level 3	0	20	41	53
Transfer out of Level 3	(9)	(10)	(35)	(10)
Financial assets, Ending Balance	467	449	467	449
Corporate and other
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	689	597	633	546
Impact of accounting change				0
Total realized/unrealized gains (losses) included in Net income	0	0	0	(3)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	(3)	7	24	(4)
Purchases and issuances	37	24	131	103
Sales and settlements	(14)	(13)	(89)	(37)
Transfer into Level 3	0	31	15	41
Transfer out of Level 3	0	0	(5)	0
Financial assets, Ending Balance	709	646	709	646
Equity securities
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	166	118	140	93
Impact of accounting change				0
Total realized/unrealized gains (losses) included in Net income	5	0	(12)	(4)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	5	(2)	21	(1)
Purchases and issuances	10	7	22	52
Sales and settlements	(21)	0	(21)	0
Transfer into Level 3	0	0	15	0
Transfer out of Level 3	0	0	0	(17)
Financial assets, Ending Balance	165	123	165	123
Assets of MIE
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	26	29	26	31
Impact of accounting change				0
Total realized/unrealized gains (losses) included in Net income	(2)	(3)	(6)	(9)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	0	0	0
Purchases and issuances	0	0	4	4
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	$ 24	$ 26	$ 24	$ 26