Fair Value Measurements - Fair Value Recurring Basis, Unobservable Input (Details) - Significant Unobservable Inputs (Level 3) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Liabilities
Changes in Unrealized Gain/(Loss) Included in Net Income, Liabilities	$ (1)	$ (1)	$ (3)	$ (3)
Contingent Consideration
Liabilities
Changes in Unrealized Gain/(Loss) Included in Net Income, Liabilities	(1)	(1)	(3)	(3)
Fair Value, Measurements, Recurring
Assets
Beginning balance	2,021	2,229	2,030	2,202
Total realized/unrealized gains (losses), Included in net income	(1)	(2)	30	(8)
Total realized/unrealized gains (losses), Included in OCI	(4)	21	(20)	55
Purchases	274	111	751	661
Settlements	(82)	(79)	(294)	(351)
Sales	(38)	(30)	(124)	(204)
Transfers into Level 3	9	23	27	81
Transfers out of Level 3	(229)	(184)	(450)	(347)
Ending balance	1,950	2,089	1,950	2,089
Freestanding Derivatives, net
Beginning balance	3	5	2	10
Total realized/unrealized gains (losses), Included in net income	0	(1)	2	(11)
Total realized/unrealized gains (losses), Included in OCI	0	0	0	0
Purchases	0	0	1	5
Settlements	0	0	0	0
Sales	0	0	(2)	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Ending balance	3	4	3	4
Liabilities
Beginning balance	(31)	(27)	(29)	(25)
Total realized/unrealized gains (losses), Included in OCI	0	0	0	0
Purchases	0	0	0	0
Settlements	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Sales	0	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Ending balance	(32)	(28)	(32)	(28)
Fair Value, Measurements, Recurring | Contingent Consideration
Liabilities
Beginning balance	(31)	(27)	(29)	(25)
Total realized/unrealized gains (losses), Included in OCI	0	0	0	0
Purchases	0	0	0	0
Settlements	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Sales	0	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Ending balance	(32)	(28)	(32)	(28)
Fair Value, Measurements, Recurring | Equity
Freestanding Derivatives, net
Beginning balance	1	2	1	0
Total realized/unrealized gains (losses), Included in net income	0	0	1	(3)
Total realized/unrealized gains (losses), Included in OCI	0	0	0	0
Purchases	0	0	1	5
Settlements	0	0	0	0
Sales	0	0	(2)	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Ending balance	1	2	1	2
Fair Value, Measurements, Recurring | Interest rate derivatives
Freestanding Derivatives, net
Beginning balance	2	3	1	9
Total realized/unrealized gains (losses), Included in net income	0	(1)	1	(7)
Total realized/unrealized gains (losses), Included in OCI	0	0	0	0
Purchases	0	0	0	0
Settlements	0	0	0	0
Sales	0	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Ending balance	2	2	2	2
Fair Value, Measurements, Recurring | Other contracts
Freestanding Derivatives, net
Beginning balance				1
Total realized/unrealized gains (losses), Included in net income				(1)
Total realized/unrealized gains (losses), Included in OCI				0
Purchases				0
Settlements				0
Sales				0
Transfers into Level 3				0
Transfers out of Level 3				0
Ending balance		0		0
Fair Value, Measurements, Recurring | Equity Securities, at fair value | Equity Securities, at fair value
Assets
Beginning balance	66		76
Total realized/unrealized gains (losses), Included in net income	0		28
Total realized/unrealized gains (losses), Included in OCI	0		1
Purchases	12		13
Settlements	0		0
Sales	0		(40)
Transfers into Level 3	0		0
Transfers out of Level 3	0		0
Ending balance	78		78
Fair Value, Measurements, Recurring | Fixed Income Securities, Fair Value Option [Member] | Total fixed maturities, FVO
Assets
Beginning balance				11
Total realized/unrealized gains (losses), Included in net income				0
Total realized/unrealized gains (losses), Included in OCI				0
Purchases				4
Settlements				(2)
Sales				(13)
Transfers into Level 3				0
Transfers out of Level 3				0
Ending balance		0		0
Fair Value, Measurements, Recurring | Securities available-for-sale and other | Equity Securities, at fair value
Assets
Beginning balance		55		55
Total realized/unrealized gains (losses), Included in net income		0		0
Total realized/unrealized gains (losses), Included in OCI		(2)		(4)
Purchases		20		22
Settlements		0		0
Sales		0		0
Transfers into Level 3		0		0
Transfers out of Level 3		0		0
Ending balance		73		73
Fair Value, Measurements, Recurring | Securities available-for-sale and other | Total fixed maturities, FVO
Assets
Beginning balance	1,952	2,169	1,952	2,126
Total realized/unrealized gains (losses), Included in net income	(1)	(1)	0	3
Total realized/unrealized gains (losses), Included in OCI	(4)	23	(21)	59
Purchases	262	91	737	630
Settlements	(82)	(79)	(294)	(349)
Sales	(38)	(30)	(82)	(191)
Transfers into Level 3	9	23	27	81
Transfers out of Level 3	(229)	(184)	(450)	(347)
Ending balance	1,869	2,012	1,869	2,012
Fair Value, Measurements, Recurring | Securities available-for-sale and other | ABS | Total fixed maturities, FVO
Assets
Beginning balance	57	63	19	45
Total realized/unrealized gains (losses), Included in net income	0	0	0	0
Total realized/unrealized gains (losses), Included in OCI	0	0	0	0
Purchases	39	0	89	56
Settlements	(2)	(3)	(5)	(6)
Sales	0	(6)	0	(6)
Transfers into Level 3	9	4	12	27
Transfers out of Level 3	(49)	(32)	(61)	(90)
Ending balance	54	26	54	26
Fair Value, Measurements, Recurring | Securities available-for-sale and other | CLOs | Total fixed maturities, FVO
Assets
Beginning balance	159	203	95	154
Total realized/unrealized gains (losses), Included in net income	0	0	0	0
Purchases	211	9	309	195
Settlements	0	0	0	(101)
Sales	0	0	(4)	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	(74)	(116)	(104)	(152)
Ending balance	296	96	296	96
Fair Value, Measurements, Recurring | Securities available-for-sale and other | CMBS | Total fixed maturities, FVO
Assets
Beginning balance	28	65	69	59
Total realized/unrealized gains (losses), Included in net income	(1)	(1)	(1)	(2)
Total realized/unrealized gains (losses), Included in OCI	1	0	0	0
Purchases	0	0	25	42
Settlements	(1)	(3)	(4)	(7)
Sales	0	0	(8)	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	(5)	0	(59)	(31)
Ending balance	22	61	22	61
Fair Value, Measurements, Recurring | Securities available-for-sale and other | Corporate | Total fixed maturities, FVO
Assets
Beginning balance	559	528	520	514
Total realized/unrealized gains (losses), Included in net income	0	0	1	1
Total realized/unrealized gains (losses), Included in OCI	(2)	5	(10)	16
Purchases	12	36	143	169
Settlements	(2)	(1)	(34)	(42)
Sales	(12)	(24)	(43)	(141)
Transfers into Level 3	0	19	15	54
Transfers out of Level 3	(4)	(30)	(41)	(38)
Ending balance	551	533	551	533
Fair Value, Measurements, Recurring | Securities available-for-sale and other | Foreign government/government agencies | Total fixed maturities, FVO
Assets
Beginning balance	3	22	2	47
Total realized/unrealized gains (losses), Included in net income	0	0	0	0
Total realized/unrealized gains (losses), Included in OCI	0	1	0	3
Purchases	0	7	1	12
Settlements	0	0	0	0
Sales	0	0	0	(2)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	(5)	0	(35)
Ending balance	3	25	3	25
Fair Value, Measurements, Recurring | Securities available-for-sale and other | Municipal | Total fixed maturities, FVO
Assets
Beginning balance	9	16	17	46
Total realized/unrealized gains (losses), Included in net income	0	0	0	4
Total realized/unrealized gains (losses), Included in OCI	0	0	(1)	1
Purchases	0	0	0	0
Settlements	0	0	0	0
Sales	0	0	0	(35)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	(7)	0
Ending balance	9	16	9	16
Fair Value, Measurements, Recurring | Securities available-for-sale and other | RMBS | Total fixed maturities, FVO
Assets
Beginning balance	1,137	1,272	1,230	1,261
Total realized/unrealized gains (losses), Included in net income	0	0	0	0
Total realized/unrealized gains (losses), Included in OCI	(3)	17	(10)	39
Purchases	0	39	170	156
Settlements	(77)	(72)	(251)	(193)
Sales	(26)	0	(27)	(7)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	(97)	(1)	(178)	(1)
Ending balance	$ 934	$ 1,255	$ 934	$ 1,255