Fair Value Measurements - Changes in balances of Level 3 financial assets carried at fair value (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	$ 2,290	$ 1,762
Total realized/unrealized gains (losses) included in Net income	(13)	(5)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	(12)	14
Purchases and issuances	466	239
Sales and settlements	(79)	(57)
Transfer into Level 3	36	57
Transfer out of Level 3	(88)	(34)
Financial assets, Ending Balance	2,600	1,976
Fixed maturities
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	2,102	1,559
Total realized/unrealized gains (losses) included in Net income	(6)	2
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	(12)	7
Purchases and issuances	454	225
Sales and settlements	(75)	(57)
Transfer into Level 3	7	57
Transfer out of Level 3	(88)	(16)
Financial assets, Ending Balance	2,382	1,777
U.S. government agency
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	8	8
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	8
State and municipal
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	148	140
Total realized/unrealized gains (losses) included in Net income	0	0
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	(1)	3
Purchases and issuances	0	0
Sales and settlements	(1)	0
Transfer into Level 3	0	0
Transfer out of Level 3	(84)	0
Financial assets, Ending Balance	62	143
Residential MBS
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	122	190
Total realized/unrealized gains (losses) included in Net income	(4)	1
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	0
Purchases and issuances	0	1
Sales and settlements	(6)	(8)
Transfer into Level 3	7	7
Transfer out of Level 3	(4)	(16)
Financial assets, Ending Balance	115	175
Commercial MBS
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	36	25
Total realized/unrealized gains (losses) included in Net income	(1)	0
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	0
Purchases and issuances	12	0
Sales and settlements	0	0
Transfer into Level 3	0	4
Transfer out of Level 3	0	0
Financial assets, Ending Balance	47	29
Asset-backed securities
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	744	484
Total realized/unrealized gains (losses) included in Net income	(2)	0
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	3	0
Purchases and issuances	204	104
Sales and settlements	(37)	(11)
Transfer into Level 3	0	17
Transfer out of Level 3	0	0
Financial assets, Ending Balance	912	594
Corporate and other
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	1,044	712
Total realized/unrealized gains (losses) included in Net income	1	1
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	(14)	4
Purchases and issuances	238	120
Sales and settlements	(31)	(38)
Transfer into Level 3	0	29
Transfer out of Level 3	0	0
Financial assets, Ending Balance	1,238	828
Equity securities
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	165	174
Total realized/unrealized gains (losses) included in Net income	(5)	(6)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	7
Purchases and issuances	9	12
Sales and settlements	(4)	0
Transfer into Level 3	29	0
Transfer out of Level 3	0	(14)
Financial assets, Ending Balance	194	173
Assets of MIE
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	23	29
Total realized/unrealized gains (losses) included in Net income	(2)	(1)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	0
Purchases and issuances	3	2
Sales and settlements	0	0
Transfer into Level 3	0	0
Transfer out of Level 3	0	(4)
Financial assets, Ending Balance	$ 24	$ 26