Fair Value Measurements - Fair Value Recurring Basis, Unobservable Input (Details) - Significant Unobservable Inputs (Level 3) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Assets
Beginning balance	$ 2,202	$ 1,771
Total realized/unrealized gains (losses), Included in net income	8	(30)
Total realized/unrealized gains (losses), Included in OCI	43	22
Purchases	833	853
Settlements	(463)	(335)
Sales	(254)	(147)
Transfers into Level 3	98	293
Transfers out of Level 3	(437)	(225)
Ending balance	2,030	2,202
Freestanding Derivatives, net
Beginning balance	10	14
Total realized/unrealized gains (losses), Included in net income	(13)	(12)
Total realized/unrealized gains (losses), Included in OCI	0	0
Purchases	5	8
Settlements	0	0
Sales	0	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Ending balance	2	10
Liabilities
Beginning balance	(25)	0
Total realized/unrealized gains (losses), Included in Net Income	(4)	(2)
Total realized/unrealized gains (losses), Included in OCI	0	0
Purchases	0	(23)
Settlements	0	0
Sales	0	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Ending balance	(29)	(25)
Contingent Consideration
Liabilities
Beginning balance	(25)	0
Total realized/unrealized gains (losses), Included in Net Income	(4)	(2)
Total realized/unrealized gains (losses), Included in OCI	0	0
Purchases	0	(23)
Settlements	0	0
Sales	0	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Ending balance	(29)	(25)
Equity
Freestanding Derivatives, net
Beginning balance	0	0
Total realized/unrealized gains (losses), Included in net income	(4)	(8)
Total realized/unrealized gains (losses), Included in OCI	0	0
Purchases	5	8
Settlements	0	0
Sales	0	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Ending balance	1	0
Interest rate derivatives
Freestanding Derivatives, net
Beginning balance	9	7
Total realized/unrealized gains (losses), Included in net income	(8)	2
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	1	9
Other contracts
Freestanding Derivatives, net
Beginning balance	1	7
Total realized/unrealized gains (losses), Included in net income	(1)	(6)
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	1
Fixed Maturities, FVO | Total fixed maturities
Assets
Beginning balance	11	14
Total realized/unrealized gains (losses), Included in net income	0	(1)
Total realized/unrealized gains (losses), Included in OCI	0	0
Purchases	4	14
Settlements	(2)	(4)
Sales	(13)	(3)
Transfers into Level 3	0	0
Transfers out of Level 3	0	(9)
Ending balance	0	11
Securities available-for-sale and other | Equity securities
Assets
Beginning balance	55	55
Total realized/unrealized gains (losses), Included in net income	0	(1)
Total realized/unrealized gains (losses), Included in OCI	(3)	4
Purchases	24	2
Settlements	0	0
Sales	0	(5)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Ending balance	76	55
Securities available-for-sale and other | Total fixed maturities
Assets
Beginning balance	2,126	1,688
Total realized/unrealized gains (losses), Included in net income	21	(16)
Total realized/unrealized gains (losses), Included in OCI	46	18
Purchases	800	829
Settlements	(461)	(331)
Sales	(241)	(139)
Transfers into Level 3	98	293
Transfers out of Level 3	(437)	(216)
Ending balance	1,952	2,126
Securities available-for-sale and other | ABS | Total fixed maturities
Assets
Beginning balance	45	32
Total realized/unrealized gains (losses), Included in net income	0	0
Total realized/unrealized gains (losses), Included in OCI	0	(1)
Purchases	56	33
Settlements	(6)	(6)
Sales	(6)	0
Transfers into Level 3	27	16
Transfers out of Level 3	(97)	(29)
Ending balance	19	45
Securities available-for-sale and other | CDOs | Total fixed maturities
Assets
Beginning balance	154	211
Total realized/unrealized gains (losses), Included in net income	18	0
Total realized/unrealized gains (losses), Included in OCI	(13)	9
Purchases	214	36
Settlements	(101)	(102)
Sales	(24)	0
Transfers into Level 3	0
Transfers out of Level 3	(153)	0
Ending balance	95	154
Securities available-for-sale and other | CMBS | Total fixed maturities
Assets
Beginning balance	59	88
Total realized/unrealized gains (losses), Included in net income	(2)	(4)
Total realized/unrealized gains (losses), Included in OCI	0	(1)
Purchases	76	45
Settlements	(9)	(15)
Sales	(10)	(1)
Transfers into Level 3	0	1
Transfers out of Level 3	(45)	(54)
Ending balance	69	59
Securities available-for-sale and other | Corporate | Total fixed maturities
Assets
Beginning balance	514	320
Total realized/unrealized gains (losses), Included in net income	1	(12)
Total realized/unrealized gains (losses), Included in OCI	19	1
Purchases	232	197
Settlements	(76)	(34)
Sales	(157)	(102)
Transfers into Level 3	71	265
Transfers out of Level 3	(84)	(121)
Ending balance	520	514
Securities available-for-sale and other | Foreign government | Total fixed maturities
Assets
Beginning balance	47	43
Total realized/unrealized gains (losses), Included in net income	0	1
Total realized/unrealized gains (losses), Included in OCI	3	2
Purchases	12	16
Settlements	(1)	0
Sales	(2)	(15)
Transfers into Level 3	0	0
Transfers out of Level 3	(57)	0
Ending balance	2	47
Securities available-for-sale and other | Municipal | Total fixed maturities
Assets
Beginning balance	46	0
Total realized/unrealized gains (losses), Included in net income	4	0
Total realized/unrealized gains (losses), Included in OCI	1	(1)
Purchases	1	39
Settlements	0	0
Sales	(35)	0
Transfers into Level 3	0	8
Transfers out of Level 3	0	0
Ending balance	17	46
Securities available-for-sale and other | RMBS | Total fixed maturities
Assets
Beginning balance	1,261	994
Total realized/unrealized gains (losses), Included in net income	0	(1)
Total realized/unrealized gains (losses), Included in OCI	36	9
Purchases	209	463
Settlements	(268)	(174)
Sales	(7)	(21)
Transfers into Level 3	0	3
Transfers out of Level 3	(1)	(12)
Ending balance	$ 1,230	$ 1,261