Derivative Financial Instruments - Volume and Fair Value Positions of Derivative Instruments (Details) $ in Millions	Sep. 30, 2018 USD ($) contract	Dec. 31, 2017 USD ($) contract
Derivatives, Fair Value
Total asset derivatives, Notional amount	$ 296	$ 101
Total liability derivatives, Notional amount	2,186	2,525
Total derivatives, Notional amount	$ 2,482	$ 2,626
Total asset derivatives, Number of contracts | contract	3,979	4,485
Total liability derivatives, Number of contracts | contract	3,283	4,464
Total derivatives, Number of contracts | contract	7,262	8,949
Total asset derivatives	$ 103	$ 115
Total liability derivatives	(298)	(343)
Total derivatives, Fair value, net	(195)	(228)
Asset derivatives, Gross asset	105	115
Asset derivatives, Gross liability	(2)	0
Liability derivatives, Gross asset	1	3
Liability derivatives, Gross liability	(299)	(346)
Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, Notional amount		$ 2,506
Total liability derivatives, Number of contracts | contract		4,464
Total liability derivatives		$ (345)
Liability derivatives, Gross asset		1
Liability derivatives, Gross liability		(346)
Foreign currency swap agreements | Derivatives designated as accounting hedging instruments | Other liabilities & accrued expenses
Derivatives, Fair Value
Total liability derivatives, Notional amount		19
Total liability derivatives		2
Liability derivatives, Gross asset		2
Liability derivatives, Gross liability		0
Interest rate cap agreements | Derivatives not designated as accounting hedging instruments | Other investments
Derivatives, Fair Value
Total asset derivatives, Notional amount	20	15
Total asset derivatives	0	0
Asset derivatives, Gross asset	0	0
Asset derivatives, Gross liability	0	0
Interest rate cap agreements | Derivatives not designated as accounting hedging instruments | Other liabilities & accrued expenses
Derivatives, Fair Value
Total liability derivatives, Notional amount	18	30
Total liability derivatives	1	1
Liability derivatives, Gross asset	1	1
Liability derivatives, Gross liability	0	0
Options | Derivatives not designated as accounting hedging instruments | Other investments
Derivatives, Fair Value
Total asset derivatives, Notional amount	$ 0	$ 0
Total asset derivatives, Number of contracts | contract	620	4,485
Total asset derivatives	$ 0	$ 114
Asset derivatives, Gross asset	0	114
Asset derivatives, Gross liability	0	0
Options | Derivatives not designated as accounting hedging instruments | Other liabilities & accrued expenses
Derivatives, Fair Value
Total liability derivatives, Notional amount	$ 0
Total liability derivatives, Number of contracts | contract	0
Total liability derivatives	$ 0
Liability derivatives, Gross asset	0
Liability derivatives, Gross liability	0
Options | Derivatives not designated as accounting hedging instruments | Other investments
Derivatives, Fair Value
Total asset derivatives, Notional amount	$ 0
Total asset derivatives, Number of contracts | contract	3,320
Total asset derivatives	$ 99
Asset derivatives, Gross asset	99
Asset derivatives, Gross liability	0
Options | Derivatives not designated as accounting hedging instruments | Other liabilities & accrued expenses
Derivatives, Fair Value
Total liability derivatives, Notional amount	$ 0
Total liability derivatives, Number of contracts | contract	3,248
Total liability derivatives	$ (37)
Liability derivatives, Gross asset	0
Liability derivatives, Gross liability	(37)
Futures | Derivatives not designated as accounting hedging instruments | Other assets
Derivatives, Fair Value
Total asset derivatives, Notional amount	$ 0
Total asset derivatives, Number of contracts | contract	39
Total asset derivatives	$ 0
Asset derivatives, Gross asset	0
Asset derivatives, Gross liability	0
Futures | Derivatives not designated as accounting hedging instruments | Other liabilities & accrued expenses
Derivatives, Fair Value
Total liability derivatives, Notional amount	$ 0
Total liability derivatives, Number of contracts | contract	35
Total liability derivatives	$ 0
Liability derivatives, Gross asset	0
Liability derivatives, Gross liability	0
Options and futures | Derivatives not designated as accounting hedging instruments | Other liabilities & accrued expenses
Derivatives, Fair Value
Total liability derivatives, Notional amount		$ 0
Total liability derivatives, Number of contracts | contract		4,464
Total liability derivatives		$ (53)
Liability derivatives, Gross asset		0
Liability derivatives, Gross liability		(53)
Foreign currency forwards | Derivatives not designated as accounting hedging instruments | Other investments
Derivatives, Fair Value
Total asset derivatives, Notional amount	212
Total asset derivatives	5
Asset derivatives, Gross asset	6
Asset derivatives, Gross liability	(1)
Foreign currency forwards | Derivatives not designated as accounting hedging instruments | Other liabilities & accrued expenses
Derivatives, Fair Value
Total liability derivatives, Notional amount		207
Total liability derivatives		(8)
Liability derivatives, Gross asset		0
Liability derivatives, Gross liability		(8)
Credit default swaps – buying protection | Derivatives not designated as accounting hedging instruments | Other investments
Derivatives, Fair Value
Total asset derivatives, Notional amount	29	3
Total asset derivatives	(1)	0
Asset derivatives, Gross asset	0	0
Asset derivatives, Gross liability	(1)	0
Credit default swaps – buying protection | Derivatives not designated as accounting hedging instruments | Other liabilities & accrued expenses
Derivatives, Fair Value
Total liability derivatives, Notional amount		34
Total liability derivatives		(1)
Liability derivatives, Gross asset		0
Liability derivatives, Gross liability		(1)
Credit default swaps – selling protection | Derivatives not designated as accounting hedging instruments | Other investments
Derivatives, Fair Value
Total asset derivatives, Notional amount		80
Total asset derivatives		1
Asset derivatives, Gross asset		1
Asset derivatives, Gross liability		0
Credit default swaps – selling protection | Derivatives not designated as accounting hedging instruments | Other liabilities & accrued expenses
Derivatives, Fair Value
Total liability derivatives, Notional amount	1	1
Total liability derivatives	0	0
Liability derivatives, Gross asset	0	0
Liability derivatives, Gross liability	0	0
Other | Derivatives not designated as accounting hedging instruments | Other assets
Derivatives, Fair Value
Total asset derivatives, Notional amount	3	3
Total asset derivatives	0	0
Asset derivatives, Gross asset	0	0
Asset derivatives, Gross liability	0	0
Total return swap agreements - fixed income | Derivatives not designated as accounting hedging instruments | Other investments
Derivatives, Fair Value
Total asset derivatives, Notional amount	23
Total asset derivatives	0
Asset derivatives, Gross asset	0
Asset derivatives, Gross liability	0
Total return swap agreements - equity | Derivatives not designated as accounting hedging instruments | Other investments
Derivatives, Fair Value
Total asset derivatives, Notional amount	9
Total asset derivatives	0
Asset derivatives, Gross asset	0
Asset derivatives, Gross liability	0
Guaranteed accumulation benefits | Derivatives not designated as accounting hedging instruments | Contractholder funds
Derivatives, Fair Value
Total liability derivatives, Notional amount	200	225
Total liability derivatives	(17)	(22)
Liability derivatives, Gross asset	0	0
Liability derivatives, Gross liability	(17)	(22)
Guaranteed withdrawal benefits | Derivatives not designated as accounting hedging instruments | Contractholder funds
Derivatives, Fair Value
Total liability derivatives, Notional amount	248	274
Total liability derivatives	(10)	(12)
Liability derivatives, Gross asset	0	0
Liability derivatives, Gross liability	(10)	(12)
Equity-indexed and forward starting options in life and annuity product contracts | Derivatives not designated as accounting hedging instruments | Contractholder funds
Derivatives, Fair Value
Total liability derivatives, Notional amount	1,719	1,735
Total liability derivatives	(235)	(250)
Liability derivatives, Gross asset	0	0
Liability derivatives, Gross liability	$ (235)	$ (250)