Fair Value Measurements - Changes in balances of Level 3 financial assets carried at fair value (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	$ 1,636	$ 1,355	$ 891
Impact of accounting change		0
Total realized/unrealized gains (losses) included in Net income	(35)	(31)	5
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	21	(28)	16
Purchases and issuances	347	543	312
Sales and settlements	(217)	(183)	(154)
Transfer into Level 3	139	186	449
Transfer out of Level 3	(129)	(160)	(164)
Sale of subsidiaries		(46)
Financial assets, Ending Balance	1,762	1,636	1,355
Fixed maturities
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	1,470	1,231	830
Impact of accounting change		0
Total realized/unrealized gains (losses) included in Net income	(14)	(16)	7
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	(14)	(19)	14
Purchases and issuances	291	460	244
Sales and settlements	(189)	(183)	(143)
Transfer into Level 3	124	186	427
Transfer out of Level 3	(109)	(143)	(148)
Sale of subsidiaries		(46)
Financial assets, Ending Balance	1,559	1,470	1,231
U.S. government agency
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	15	15	15
Impact of accounting change		0
Total realized/unrealized gains (losses) included in Net income	(8)	0	0
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	1	0	0
Purchases and issuances	0	0	0
Sales and settlements	0	0	0
Transfer into Level 3	0	0	0
Transfer out of Level 3	0	0	0
Sale of subsidiaries		0
Financial assets, Ending Balance	8	15	15
State and municipal
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	89	100	61
Impact of accounting change		0
Total realized/unrealized gains (losses) included in Net income	0	0	(1)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	(4)	(1)	0
Purchases and issuances	57	34	30
Sales and settlements	(2)	(1)	0
Transfer into Level 3	0	2	10
Transfer out of Level 3	0	(39)	0
Sale of subsidiaries		(6)
Financial assets, Ending Balance	140	89	100
Residential MBS
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	224	300	316
Impact of accounting change		0
Total realized/unrealized gains (losses) included in Net income	(4)	(11)	4
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	(2)	(1)	3
Purchases and issuances	8	0	13
Sales and settlements	(28)	(34)	(31)
Transfer into Level 3	34	86	111
Transfer out of Level 3	(42)	(88)	(116)
Sale of subsidiaries		(28)
Financial assets, Ending Balance	190	224	300
Commercial MBS
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	39	44	28
Impact of accounting change		0
Total realized/unrealized gains (losses) included in Net income	(1)	(1)	(1)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	(1)	0
Purchases and issuances	0	0	0
Sales and settlements	(7)	(2)	0
Transfer into Level 3	0	4	17
Transfer out of Level 3	(6)	(1)	0
Sale of subsidiaries		(4)
Financial assets, Ending Balance	25	39	44
Asset-backed securities
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	470	226	75
Impact of accounting change		0
Total realized/unrealized gains (losses) included in Net income	(1)	1	3
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	1	(7)	(2)
Purchases and issuances	50	265	99
Sales and settlements	(52)	(56)	(39)
Transfer into Level 3	60	53	117
Transfer out of Level 3	(44)	(10)	(27)
Sale of subsidiaries		(2)
Financial assets, Ending Balance	484	470	226
Corporate and other
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	633	546	335
Impact of accounting change		0
Total realized/unrealized gains (losses) included in Net income	0	(5)	2
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	(10)	(9)	13
Purchases and issuances	176	161	102
Sales and settlements	(100)	(90)	(73)
Transfer into Level 3	30	41	172
Transfer out of Level 3	(17)	(5)	(5)
Sale of subsidiaries		(6)
Financial assets, Ending Balance	712	633	546
Equity securities
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	140	93	31
Impact of accounting change		0
Total realized/unrealized gains (losses) included in Net income	(12)	(4)	1
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	35	(9)	2
Purchases and issuances	44	77	62
Sales and settlements	(28)	0	(9)
Transfer into Level 3	15	0	22
Transfer out of Level 3	(20)	(17)	(16)
Sale of subsidiaries		0
Financial assets, Ending Balance	174	140	93
Assets of MIE
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	26	31	30
Impact of accounting change		0
Total realized/unrealized gains (losses) included in Net income	(9)	(11)	(3)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	0	0
Purchases and issuances	12	6	6
Sales and settlements	0	0	(2)
Transfer into Level 3	0	0	0
Transfer out of Level 3	0	0	0
Sale of subsidiaries		0
Financial assets, Ending Balance	$ 29	$ 26	$ 31