Fair Value Measurements - Fair Value Recurring Basis, Unobservable Input (Details) - Significant Unobservable Inputs (Level 3) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Liabilities
Changes in Unrealized Gain/(Loss) Included in Net Income, Liabilities	$ (4)	$ (1)	$ (2)	$ (2)
Contingent Consideration
Liabilities
Changes in Unrealized Gain/(Loss) Included in Net Income, Liabilities	(4)	(1)	(2)	(2)
Fair Value, Measurements, Recurring
Assets
Beginning balance	1,987	2,242	2,030	2,202
Total realized/unrealized gains (losses), Included in net income	(1)	0	31	(6)
Total realized/unrealized gains (losses), Included in OCI	(10)	19	(16)	34
Purchases	289	290	477	550
Settlements	(114)	(172)	(212)	(272)
Sales	(22)	(52)	(86)	(174)
Transfers into Level 3	18	0	18	58
Transfers out of Level 3	(126)	(98)	(221)	(163)
Ending balance	2,021	2,229	2,021	2,229
Freestanding Derivatives, net
Beginning balance	3	9	2	10
Total realized/unrealized gains (losses), Included in net income	(1)	(4)	2	(10)
Total realized/unrealized gains (losses), Included in OCI	0	0	0	0
Purchases	1	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	5	3	5
Liabilities
Beginning balance	(27)	(26)	(29)	(25)
Total realized/unrealized gains (losses), Included in Net Income	(4)	(1)	(2)	(2)
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	(31)	(27)	(31)	(27)
Fair Value, Measurements, Recurring | Contingent Consideration
Liabilities
Beginning balance	(27)	(26)	(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	(31)	(27)	(31)	(27)
Fair Value, Measurements, Recurring | Equity derivatives
Freestanding Derivatives, net
Beginning balance	1	4	1	0
Total realized/unrealized gains (losses), Included in net income	(1)	(2)	1	(3)
Total realized/unrealized gains (losses), Included in OCI	0	0	0	0
Purchases	1	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	5	1	9
Total realized/unrealized gains (losses), Included in net income	0		1	(6)
Total realized/unrealized gains (losses), Included in OCI	0		0	0
Purchases	0		0	0
Settlements	0		0	0
Sales	0		0	0
Transfers into Level 3	0		0	0
Transfers out of Level 3	0		0	0
Ending balance	2	3	2	3
Fair Value, Measurements, Recurring | Other contracts
Freestanding Derivatives, net
Beginning balance		0		1
Total realized/unrealized gains (losses), Included in net income		0		(1)
Total realized/unrealized gains (losses), Included in OCI		0		0
Purchases		0		0
Settlements		0		0
Sales		0		0
Transfers into Level 3		0		0
Transfers out of Level 3		0		0
Ending balance		0		0
Fair Value, Measurements, Recurring | Equity Securities, at fair value | Equity Securities, at fair value
Assets
Beginning balance	65		76
Total realized/unrealized gains (losses), Included in net income	0		28
Total realized/unrealized gains (losses), Included in OCI	1		1
Purchases	1		1
Settlements	0		0
Sales	(1)		(40)
Transfers into Level 3	0		0
Transfers out of Level 3	0		0
Ending balance	66		66
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		0		(2)
Purchases		0		2
Settlements		0		0
Sales		0		0
Transfers into Level 3		0		0
Transfers out of Level 3		0		0
Ending balance		55		55
Fair Value, Measurements, Recurring | Securities available-for-sale and other | Total fixed maturities, FVO
Assets
Beginning balance	1,919	2,178	1,952	2,126
Total realized/unrealized gains (losses), Included in net income	0	4	1	4
Total realized/unrealized gains (losses), Included in OCI	(11)	19	(17)	36
Purchases	287	290	475	539
Settlements	(114)	(172)	(212)	(270)
Sales	(21)	(52)	(44)	(161)
Transfers into Level 3	18	0	18	58
Transfers out of Level 3	(126)	(98)	(221)	(163)
Ending balance	1,952	2,169	1,952	2,169
Fair Value, Measurements, Recurring | Securities available-for-sale and other | ABS | Total fixed maturities, FVO
Assets
Beginning balance	14	81	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	50	23	50	56
Settlements	(1)	0	(3)	(3)
Sales	0	0	0	0
Transfers into Level 3	3	0	3	23
Transfers out of Level 3	(9)	(41)	(12)	(58)
Ending balance	57	63	57	63
Fair Value, Measurements, Recurring | Securities available-for-sale and other | CDOs | Total fixed maturities, FVO
Assets
Beginning balance	106	120	95	154
Total realized/unrealized gains (losses), Included in net income	0	0	0	0
Total realized/unrealized gains (losses), Included in OCI		(3)
Purchases	77	186	98	186
Settlements	0	(100)	0	(101)
Sales	(4)	0	(4)	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	(20)	0	(30)	(36)
Ending balance	159	203	159	203
Fair Value, Measurements, Recurring | Securities available-for-sale and other | CMBS | Total fixed maturities, FVO
Assets
Beginning balance	33	72	69	59
Total realized/unrealized gains (losses), Included in net income	0	0	0	(1)
Total realized/unrealized gains (losses), Included in OCI	0	0	(1)	0
Purchases	25	14	25	42
Settlements	(2)	(2)	(3)	(4)
Sales	(8)	0	(8)	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	(20)	(19)	(54)	(31)
Ending balance	28	65	28	65
Fair Value, Measurements, Recurring | Securities available-for-sale and other | Corporate | Total fixed maturities, FVO
Assets
Beginning balance	515	521	520	514
Total realized/unrealized gains (losses), Included in net income	0	0	1	1
Total realized/unrealized gains (losses), Included in OCI	(7)	6	(8)	11
Purchases	66	33	131	133
Settlements	(18)	(4)	(32)	(41)
Sales	(8)	(20)	(31)	(117)
Transfers into Level 3	15	0	15	35
Transfers out of Level 3	(4)	(8)	(37)	(8)
Ending balance	559	528	559	528
Fair Value, Measurements, Recurring | Securities available-for-sale and other | Foreign government/government agencies | Total fixed maturities, FVO
Assets
Beginning balance	2	49	2	47
Total realized/unrealized gains (losses), Included in net income	0	0	0	0
Total realized/unrealized gains (losses), Included in OCI	0	0	0	2
Purchases	1	5	1	5
Settlements	0	0	0	0
Sales	0	(2)	0	(2)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	(30)	0	(30)
Ending balance	3	22	3	22
Fair Value, Measurements, Recurring | Securities available-for-sale and other | Municipal | Total fixed maturities, FVO
Assets
Beginning balance	16	44	17	46
Total realized/unrealized gains (losses), Included in net income	0	4	0	4
Total realized/unrealized gains (losses), Included in OCI	0	(2)	(1)	1
Purchases	0	0	0	0
Settlements	0	0	0	0
Sales	0	(30)	0	(35)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	(7)	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	1,233	1,291	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	(4)	18	(7)	22
Purchases	68	29	170	117
Settlements	(93)	(66)	(174)	(121)
Sales	(1)	0	(1)	(7)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	(66)	0	(81)	0
Ending balance	$ 1,137	$ 1,272	$ 1,137	$ 1,272