Investments - Investments - Available-for-Sale Securities (Details) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, at fair value, amortized cost	$ 36,094,000,000	$ 35,612,000,000
Gross Unrealized Gains, fixed maturities, available-for-sale	679,000,000	1,466,000,000
Gross Unrealized Losses, fixed maturities, available-for-sale	(607,000,000)	(114,000,000)
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	36,166,000,000	36,964,000,000
Non-Credit OTTI	(5,000,000)	(5,000,000)
Equity securities, available-for-sale, Cost or Amortized Cost	0	907,000,000
Equity securities, AFS	0	1,012,000,000
Available-for-sale, Cost or Amortized Cost	36,094,000,000	36,519,000,000
Available-for-sale, Gross Unrealized Gains	679,000,000	1,587,000,000
Available-for-sale, Gross Unrealized Losses	(607,000,000)	(130,000,000)
Available-for-sale, Fair Value	36,166,000,000	37,976,000,000
Amortized Cost
One year or less	1,033,000,000	1,507,000,000
Over one year through five years	6,233,000,000	5,007,000,000
Over five years through ten years	6,543,000,000	6,505,000,000
Over ten years	12,922,000,000	13,928,000,000
Subtotal	26,731,000,000	26,947,000,000
Mortgage-backed and asset-backed securities	9,363,000,000	8,665,000,000
Fair Value
One year or less	1,037,000,000	1,513,000,000
Over one year through five years	6,242,000,000	5,119,000,000
Over five years through ten years	6,469,000,000	6,700,000,000
Over ten years	13,126,000,000	14,866,000,000
Subtotal	26,874,000,000	28,198,000,000
Mortgage-backed and asset-backed securities	9,292,000,000	8,766,000,000
Concentration Risk, Credit Risk, Financial Instrument, Maximum Exposure	0	0
ABS
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, at fair value, amortized cost	1,191,000,000	1,119,000,000
Gross Unrealized Gains, fixed maturities, available-for-sale	3,000,000	9,000,000
Gross Unrealized Losses, fixed maturities, available-for-sale	(3,000,000)	(2,000,000)
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	1,191,000,000	1,126,000,000
Non-Credit OTTI	0	0
CLOs
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, at fair value, amortized cost	1,328,000,000	1,257,000,000
Gross Unrealized Gains, fixed maturities, available-for-sale	1,000,000	3,000,000
Gross Unrealized Losses, fixed maturities, available-for-sale	(3,000,000)	0
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	1,326,000,000	1,260,000,000
Non-Credit OTTI	0	0
CMBS
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, at fair value, amortized cost	3,718,000,000	3,304,000,000
Gross Unrealized Gains, fixed maturities, available-for-sale	25,000,000	58,000,000
Gross Unrealized Losses, fixed maturities, available-for-sale	(86,000,000)	(26,000,000)
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	3,657,000,000	3,336,000,000
Non-Credit OTTI	(5,000,000)	(5,000,000)
Corporate
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, at fair value, amortized cost	13,640,000,000	12,370,000,000
Gross Unrealized Gains, fixed maturities, available-for-sale	176,000,000	490,000,000
Gross Unrealized Losses, fixed maturities, available-for-sale	(324,000,000)	(56,000,000)
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	13,492,000,000	12,804,000,000
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	961,000,000	1,071,000,000
Gross Unrealized Gains, fixed maturities, available-for-sale	11,000,000	43,000,000
Gross Unrealized Losses, fixed maturities, available-for-sale	(20,000,000)	(4,000,000)
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	952,000,000	1,110,000,000
Non-Credit OTTI	0	0
Municipal
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, at fair value, amortized cost	10,276,000,000	11,743,000,000
Gross Unrealized Gains, fixed maturities, available-for-sale	402,000,000	754,000,000
Gross Unrealized Losses, fixed maturities, available-for-sale	(76,000,000)	(12,000,000)
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	10,602,000,000	12,485,000,000
Non-Credit OTTI	0	0
RMBS
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale, at fair value, amortized cost	3,126,000,000	2,985,000,000
Gross Unrealized Gains, fixed maturities, available-for-sale	44,000,000	63,000,000
Gross Unrealized Losses, fixed maturities, available-for-sale	(52,000,000)	(4,000,000)
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	3,118,000,000	3,044,000,000
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	1,854,000,000	1,763,000,000
Gross Unrealized Gains, fixed maturities, available-for-sale	17,000,000	46,000,000
Gross Unrealized Losses, fixed maturities, available-for-sale	(43,000,000)	(10,000,000)
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	1,828,000,000	1,799,000,000
Non-Credit OTTI	0	0
Equity Securities, at fair value
Debt Securities, Available-for-sale [Line Items]
Non-Credit OTTI		0
Equity securities, available-for-sale, Cost or Amortized Cost		907,000,000
Gross Unrealized Gains, fixed maturities, available-for-sale		121,000,000
Gross Unrealized Losses Gains, fixed maturities, available-for-sale		(16,000,000)
Equity securities, AFS		$ 1,012,000,000