Fair Value Measurements - Changes in balances of Level 3 financial assets carried at fair value (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	$ 1,762	$ 1,636
Total realized/unrealized gains (losses) included in Net income	(5)	(20)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	14	20
Purchases and issuances	239	102
Sales and settlements	(57)	(23)
Transfer into Level 3	57	72
Transfer out of Level 3	(34)	(16)
Financial assets, Ending Balance	1,976	1,771
Fixed maturities
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	1,559	1,470
Total realized/unrealized gains (losses) included in Net income	2	(1)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	7	12
Purchases and issuances	225	90
Sales and settlements	(57)	(23)
Transfer into Level 3	57	57
Transfer out of Level 3	(16)	(16)
Financial assets, Ending Balance	1,777	1,589
U.S. government agency
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	8	15
Total realized/unrealized gains (losses) included in Net income	0	0
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	0
Purchases and issuances	0	0
Sales and settlements	0	0
Transfer into Level 3	0	0
Transfer out of Level 3	0	0
Financial assets, Ending Balance	8	15
State and municipal
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	140	89
Total realized/unrealized gains (losses) included in Net income	0	0
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	3	3
Purchases and issuances	0	0
Sales and settlements	0	0
Transfer into Level 3	0	0
Transfer out of Level 3	0	0
Financial assets, Ending Balance	143	92
Residential MBS
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	190	224
Total realized/unrealized gains (losses) included in Net income	1	1
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	0
Purchases and issuances	1	0
Sales and settlements	(8)	(7)
Transfer into Level 3	7	11
Transfer out of Level 3	(16)	(16)
Financial assets, Ending Balance	175	213
Commercial MBS
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	25	39
Total realized/unrealized gains (losses) included in Net income	0	0
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	0
Purchases and issuances	0	0
Sales and settlements	0	(1)
Transfer into Level 3	4	0
Transfer out of Level 3	0	0
Financial assets, Ending Balance	29	38
Asset-backed securities
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	484	470
Total realized/unrealized gains (losses) included in Net income	0	0
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	(6)
Purchases and issuances	104	4
Sales and settlements	(11)	(8)
Transfer into Level 3	17	41
Transfer out of Level 3	0	0
Financial assets, Ending Balance	594	501
Corporate and other
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	712	633
Total realized/unrealized gains (losses) included in Net income	1	(2)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	4	15
Purchases and issuances	120	86
Sales and settlements	(38)	(7)
Transfer into Level 3	29	5
Transfer out of Level 3	0	0
Financial assets, Ending Balance	828	730
Equity securities
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	174	140
Total realized/unrealized gains (losses) included in Net income	(6)	(17)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	7	8
Purchases and issuances	12	12
Sales and settlements	0	0
Transfer into Level 3	0	15
Transfer out of Level 3	(14)	0
Financial assets, Ending Balance	173	158
Assets of MIE
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	29	26
Total realized/unrealized gains (losses) included in Net income	(1)	(2)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	0
Purchases and issuances	2	0
Sales and settlements	0	0
Transfer into Level 3	0	0
Transfer out of Level 3	(4)	0
Financial assets, Ending Balance	$ 26	$ 24