Fair Value Measurements - Changes in balances of Level 3 financial assets carried at fair value (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	$ 2,600	$ 2,290	$ 1,976	$ 2,290	$ 1,762
Total realized/unrealized gains (losses) included in Net income	14		(13)	1	(18)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	(11)		15	(23)	29
Purchases and issuances	397		193	863	432
Sales and settlements	(73)		(79)	(152)	(136)
Transfer into Level 3	51		32	87	89
Transfer out of Level 3	(41)		(133)	(129)	(167)
Financial assets, Ending Balance	2,937	2,600	1,991	2,937	1,991
Fixed maturities
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	2,382	2,102	1,777	2,102	1,559
Total realized/unrealized gains (losses) included in Net income	(2)		2	(8)	4
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	(11)		9	(23)	16
Purchases and issuances	379		183	833	408
Sales and settlements	(73)		(76)	(148)	(133)
Transfer into Level 3	50		32	57	89
Transfer out of Level 3	(41)		(127)	(129)	(143)
Financial assets, Ending Balance	2,684	2,382	1,800	2,684	1,800
U.S. government agency
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	8	8	8	8	8
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		0	0	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	8	8	8	8
State and municipal
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	62	148	143	148	140
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	(2)	4
Purchases and issuances	0		0	0	0
Sales and settlements	0		(1)	(1)	(1)
Transfer into Level 3	0		0	0	0
Transfer out of Level 3	0		0	(84)	0
Financial assets, Ending Balance	61	62	143	61	143
Residential MBS
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	115	122	175	122	190
Total realized/unrealized gains (losses) included in Net income	(3)		(3)	(7)	(2)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0		2	0	2
Purchases and issuances	0		0	0	1
Sales and settlements	(5)		(23)	(11)	(31)
Transfer into Level 3	50		13	57	20
Transfer out of Level 3	(10)		(11)	(14)	(27)
Financial assets, Ending Balance	147	115	153	147	153
Commercial MBS
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	47	36	29	36	25
Total realized/unrealized gains (losses) included in Net income	0		1	(1)	1
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0		0	0	0
Purchases and issuances	9		15	21	15
Sales and settlements	0		0	0	0
Transfer into Level 3	0		0	0	4
Transfer out of Level 3	0		0	0	0
Financial assets, Ending Balance	56	47	45	56	45
Asset-backed securities
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	912	744	594	744	484
Total realized/unrealized gains (losses) included in Net income	0		0	(2)	0
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	(6)		2	(3)	2
Purchases and issuances	136		0	340	104
Sales and settlements	(20)		(25)	(57)	(36)
Transfer into Level 3	0		19	0	36
Transfer out of Level 3	(18)		(92)	(18)	(92)
Financial assets, Ending Balance	1,004	912	498	1,004	498
Corporate and other
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	1,238	1,044	828	1,044	712
Total realized/unrealized gains (losses) included in Net income	1		4	2	5
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	(4)		4	(18)	8
Purchases and issuances	234		168	472	288
Sales and settlements	(48)		(27)	(79)	(65)
Transfer into Level 3	0		0	0	29
Transfer out of Level 3	(13)		(24)	(13)	(24)
Financial assets, Ending Balance	1,408	1,238	953	1,408	953
Equity securities
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	194	165	173	165	174
Total realized/unrealized gains (losses) included in Net income	19		(10)	14	(16)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0		6	0	13
Purchases and issuances	16		8	25	20
Sales and settlements	0		(3)	(4)	(3)
Transfer into Level 3	1	29	0	30	0
Transfer out of Level 3	0		(6)	0	(20)
Financial assets, Ending Balance	230	194	168	230	168
Assets of MIE
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	24	23	26	23	29
Total realized/unrealized gains (losses) included in Net income	(3)		(5)	(5)	(6)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0		0	0	0
Purchases and issuances	2		2	5	4
Sales and settlements	0		0	0	0
Transfer into Level 3	0		0	0	0
Transfer out of Level 3	0		0	0	(4)
Financial assets, Ending Balance	$ 23	$ 24	$ 23	$ 23	$ 23