Fair Value Measurements - Fair Value Recurring Basis, Unobservable Input (Details) - Fair Value, Measurements, Recurring - Significant Unobservable Inputs (Level 3) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs		$ 3		$ 3			$ 3	$ 2
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Earnings		0		2
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Other Comprehensive Income (Loss)		0		0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Purchases		0		1
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Settlements		0		0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Sales		0		2
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Transfers into Level 3		0		0
Assets
Beginning balance	$ 1,727	2,021	$ 1,643	2,030
Total realized/unrealized gains (losses), Included in net income	(5)	(1)	(7)	30
Total realized/unrealized gains (losses), Included in OCI	(3)	(4)	5	(20)
Purchases	268	274	772	751
Settlements	(67)	(82)	(198)	(294)
Sales	(4)	(38)	(122)	(124)
Transfers into Level 3	15	9	61	27
Transfers out of Level 3	(205)	(229)	(428)	(450)
Ending balance	1,726	1,950	1,726	1,950
Liabilities
Changes in Unrealized Gain/(Loss) Included in Net Income, Liabilities	2	1	14	3
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Transfers out of Level 3		0		0
Contingent Consideration
Liabilities
Beginning balance	(21)	(31)	(35)	(29)
Changes in Unrealized Gain/(Loss) Included in Net Income, Liabilities	0	1	6	3
Total realized/unrealized gains (losses), Included in OCI	0	0	0	0
Purchases	0	0	0	0
Settlements	0	0	(20)	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	(21)	(32)	(21)	(32)
Liability [Member]
Liabilities
Beginning balance	(24)	(31)	(32)	(29)
Changes in Unrealized Gain/(Loss) Included in Net Income, Liabilities	2	1	14	3
Total realized/unrealized gains (losses), Included in OCI	0	0	0	0
Purchases	0	0	0	0
Settlements	0	0	(20)	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	(26)	(32)	(26)	(32)
Equity
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs		1		1			1	1
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Earnings		0		1
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Other Comprehensive Income (Loss)		0		0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Purchases		0		1
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Settlements		0		0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Sales		0		2
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Transfers into Level 3		0		0
Liabilities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Transfers out of Level 3		0		0
Equity | Liability [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	(5)		(5)		$ (3)	$ 3
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Earnings	(2)		(8)
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Other Comprehensive Income (Loss)	0		0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Purchases	0		0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Settlements	0		0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Sales	0		0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Transfers into Level 3	0		0
Liabilities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Transfers out of Level 3	0		0
Interest rate derivatives
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	0	2	0	2		1	$ 2	$ 1
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Earnings		0	(1)	1
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Other Comprehensive Income (Loss)		0	0	0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Purchases		0	0	0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Settlements		0	0	0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Sales		0	0	0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Transfers into Level 3		0	0	0
Liabilities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Transfers out of Level 3		0	0	0
Derivative [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	0		0			1
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Earnings			(1)
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Other Comprehensive Income (Loss)			0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Purchases			0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Settlements			0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Sales			0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Transfers into Level 3			0
Liabilities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Transfers out of Level 3			0
Derivative [Member] | Liability [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	(5)		(5)		$ (3)	$ 3
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Earnings	(2)		(8)
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Other Comprehensive Income (Loss)	0		0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Purchases	0		0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Settlements	0		0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Sales	0		0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Transfers into Level 3	0		0
Liabilities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Transfers out of Level 3	0		0
Securities available-for-sale and other | Total fixed maturities, FVO
Assets
Beginning balance	1,655	1,952	1,565	1,952
Total realized/unrealized gains (losses), Included in net income	(3)	(1)	(3)	0
Total realized/unrealized gains (losses), Included in OCI	(3)	(4)	5	(21)
Purchases	268	262	763	737
Settlements	(67)	(82)	(198)	(294)
Sales	(4)	(38)	(109)	(82)
Transfers into Level 3	15	9	61	27
Transfers out of Level 3	(205)	(229)	(428)	(450)
Ending balance	1,656	1,869	1,656	1,869
Securities available-for-sale and other | ABS | Total fixed maturities, FVO
Assets
Beginning balance	5	57	10	19
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	0	39	5	89
Settlements	0	(2)	(1)	(5)
Sales	0	0	0	0
Transfers into Level 3	0	9	0	12
Transfers out of Level 3	(5)	(49)	(14)	(61)
Ending balance	0	54	0	54
Securities available-for-sale and other | CLOs | Total fixed maturities, FVO
Assets
Beginning balance	286	159	100	95
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	92	211	329	309
Settlements	(8)	0	(18)	0
Sales	0	0	(6)	(4)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	(74)	(74)	(109)	(104)
Ending balance	296	296	296	296
Securities available-for-sale and other | CMBS | Total fixed maturities, FVO
Assets
Beginning balance	35	28	12	69
Total realized/unrealized gains (losses), Included in net income	0	(1)	0	(1)
Total realized/unrealized gains (losses), Included in OCI	0	1	1	0
Purchases	10	0	34	25
Settlements	(1)	(1)	(3)	(4)
Sales	0	0	0	(8)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	(24)	(5)	(24)	(59)
Ending balance	20	22	20	22
Securities available-for-sale and other | Corporate | Total fixed maturities, FVO
Assets
Beginning balance	568	559	520	520
Total realized/unrealized gains (losses), Included in net income	(3)	0	(4)	1
Total realized/unrealized gains (losses), Included in OCI	0	(2)	9	(10)
Purchases	166	12	261	143
Settlements	(7)	(2)	(13)	(34)
Sales	(4)	(12)	(68)	(43)
Transfers into Level 3	15	0	61	15
Transfers out of Level 3	(12)	(4)	(43)	(41)
Ending balance	723	551	723	551
Securities available-for-sale and other | Foreign government/government agencies | Total fixed maturities, FVO
Assets
Beginning balance	3	3	3	2
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	0	0	0	1
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	3	3	3	3
Securities available-for-sale and other | Municipal | Total fixed maturities, FVO
Assets
Beginning balance		9		17
Total realized/unrealized gains (losses), Included in net income		0		0
Total realized/unrealized gains (losses), Included in OCI		0		(1)
Purchases		0		0
Settlements		0		0
Sales		0		0
Transfers into Level 3		0		0
Transfers out of Level 3		0		(7)
Ending balance		9		9
Securities available-for-sale and other | RMBS | Total fixed maturities, FVO
Assets
Beginning balance	758	1,137	920	1,230
Total realized/unrealized gains (losses), Included in net income	0	0	1	0
Total realized/unrealized gains (losses), Included in OCI	(3)	(3)	(5)	(10)
Purchases	0	0	134	170
Settlements	(51)	(77)	(163)	(251)
Sales	0	(26)	(35)	(27)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	(90)	(97)	(238)	(178)
Ending balance	614	934	614	934
Equity Securities, at fair value | Equity Securities, at fair value
Assets
Beginning balance	72		77
Total realized/unrealized gains (losses), Included in net income	(2)		(3)
Total realized/unrealized gains (losses), Included in OCI	0		0
Purchases	0		9
Settlements	0		0
Sales	0		(13)
Transfers into Level 3	0		0
Transfers out of Level 3	0		0
Ending balance	$ 70		$ 70
Equity Securities, at fair value | Securities available-for-sale and other
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