Investments - Investments - Available-for-Sale Securities (Details) - USD ($)	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, at fair value, amortized cost	$ 35,894,000,000	$ 35,603,000,000
Gross Unrealized Gains, fixed maturities, available-for-sale	1,134,000,000	703,000,000
Gross Unrealized Losses, fixed maturities, available-for-sale	(209,000,000)	(654,000,000)
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	36,819,000,000	35,652,000,000
Non-Credit OTTI	(4,000,000)	(5,000,000)
Concentration Risk, Credit Risk, Financial Instrument, Maximum Exposure	0	0
Amortized Cost
One year or less	964,000,000	999,000,000
Over one year through five years	5,785,000,000	5,786,000,000
Over five years through ten years	7,259,000,000	6,611,000,000
Over ten years	12,444,000,000	12,629,000,000
Subtotal	26,452,000,000	26,025,000,000
Mortgage-backed and asset-backed securities	9,442,000,000	9,578,000,000
Fair Value
One year or less	969,000,000	1,002,000,000
Over one year through five years	5,871,000,000	5,791,000,000
Over five years through ten years	7,391,000,000	6,495,000,000
Over ten years	13,066,000,000	12,820,000,000
Subtotal	27,297,000,000	26,108,000,000
Mortgage-backed and asset-backed securities	9,522,000,000	9,544,000,000
ABS
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, at fair value, amortized cost	959,000,000	1,272,000,000
Gross Unrealized Gains, fixed maturities, available-for-sale	10,000,000	5,000,000
Gross Unrealized Losses, fixed maturities, available-for-sale	(1,000,000)	(1,000,000)
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	968,000,000	1,276,000,000
Non-Credit OTTI	0	0
CLOs
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, at fair value, amortized cost	1,442,000,000	1,455,000,000
Gross Unrealized Gains, fixed maturities, available-for-sale	7,000,000	2,000,000
Gross Unrealized Losses, fixed maturities, available-for-sale	(11,000,000)	(20,000,000)
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	1,438,000,000	1,437,000,000
Non-Credit OTTI	0	0
CMBS
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, at fair value, amortized cost	3,534,000,000	3,581,000,000
Gross Unrealized Gains, fixed maturities, available-for-sale	67,000,000	35,000,000
Gross Unrealized Losses, fixed maturities, available-for-sale	(33,000,000)	(64,000,000)
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	3,568,000,000	3,552,000,000
Non-Credit OTTI	(4,000,000)	(5,000,000)
Corporate
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, at fair value, amortized cost	14,187,000,000	13,696,000,000
Gross Unrealized Gains, fixed maturities, available-for-sale	349,000,000	148,000,000
Gross Unrealized Losses, fixed maturities, available-for-sale	(133,000,000)	(446,000,000)
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	14,403,000,000	13,398,000,000
Non-Credit OTTI	0	0
Foreign Govt./Govt. Agencies
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, at fair value, amortized cost	866,000,000	866,000,000
Gross Unrealized Gains, fixed maturities, available-for-sale	23,000,000	7,000,000
Gross Unrealized Losses, fixed maturities, available-for-sale	(7,000,000)	(26,000,000)
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	882,000,000	847,000,000
Non-Credit OTTI	0	0
Municipal
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, at fair value, amortized cost	9,780,000,000	9,972,000,000
Gross Unrealized Gains, fixed maturities, available-for-sale	573,000,000	421,000,000
Gross Unrealized Losses, fixed maturities, available-for-sale	(7,000,000)	(47,000,000)
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	10,346,000,000	10,346,000,000
Non-Credit OTTI	0	0
RMBS
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, at fair value, amortized cost	3,507,000,000	3,270,000,000
Gross Unrealized Gains, fixed maturities, available-for-sale	54,000,000	44,000,000
Gross Unrealized Losses, fixed maturities, available-for-sale	(13,000,000)	(35,000,000)
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	3,548,000,000	3,279,000,000
Non-Credit OTTI	0	0
U.S. Treasuries
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, at fair value, amortized cost	1,619,000,000	1,491,000,000
Gross Unrealized Gains, fixed maturities, available-for-sale	51,000,000	41,000,000
Gross Unrealized Losses, fixed maturities, available-for-sale	(4,000,000)	(15,000,000)
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	1,666,000,000	1,517,000,000
Non-Credit OTTI	$ 0	$ 0