Derivative Financial Instruments and Off-balance sheet Financial Instruments - Volume and fair value positions of derivative instruments (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) contract	Dec. 31, 2016 USD ($) contract
Derivatives, Fair Value
Assumed recoveries under sale of credit protection	$ 0
Securities pledged in the form of margin deposits	3
Total asset derivatives, Notional amount	101	$ 395
Total liability derivatives, Notional amount	2,525	2,524
Total derivatives, Notional amount	$ 2,626	$ 2,919
Total asset derivatives, Number of contracts | contract	4,485	3,923
Total liability derivatives, Number of contracts | contract	4,464	3,928
Total derivatives, Number of contracts | contract	8,949	7,851
Total asset derivatives, Fair value, net	$ 115	$ 101
Asset derivatives, Gross asset	115	103
Asset derivatives, Gross liability	0	(2)
Derivative liabilities net amount on balance sheet	(343)	(329)
Total derivatives, Fair value, net	(228)	(228)
Liability derivatives, Gross asset	3	0
Liability derivatives, Gross liability	(346)	(329)
Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, Notional amount		$ 346
Total liability derivatives, Notional amount	$ 2,506
Total asset derivatives, Number of contracts | contract		3,923
Total liability derivatives, Number of contracts | contract	4,464
Total asset derivatives, Fair value, net		$ 96
Asset derivatives, Gross asset		98
Asset derivatives, Gross liability		(2)
Derivative liabilities net amount on balance sheet	$ (345)
Liability derivatives, Gross asset	1
Liability derivatives, Gross liability	(346)
Foreign currency swap agreements | Derivatives designated as accounting hedging instruments | Other investments
Derivatives, Fair Value
Total asset derivatives, Notional amount		49
Total asset derivatives, Fair value, net		5
Asset derivatives, Gross asset		5
Asset derivatives, Gross liability		0
Foreign currency swap agreements | Derivatives designated as accounting hedging instruments | Other liabilities & accrued expenses
Derivatives, Fair Value
Total liability derivatives, Notional amount	19
Derivative liabilities net amount on balance sheet	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	15	65
Total asset derivatives, Fair value, net	0	1
Asset derivatives, Gross asset	0	1
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	30
Derivative liabilities net amount on balance sheet	1
Liability derivatives, Gross asset	1
Liability derivatives, Gross liability	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	4,485	3,917
Total asset derivatives, Fair value, net	$ 114	$ 87
Asset derivatives, Gross asset	114	87
Asset derivatives, Gross liability	0	0
Financial futures contracts | 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		6
Total asset derivatives, Fair value, net		$ 0
Asset derivatives, Gross asset		0
Asset derivatives, Gross liability		0
Foreign currency forwards | Derivatives not designated as accounting hedging instruments | Other investments
Derivatives, Fair Value
Total asset derivatives, Notional amount		173
Total asset derivatives, Fair value, net		7
Asset derivatives, Gross asset		8
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
Derivative liabilities net amount on balance sheet	(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	3	25
Total asset derivatives, Fair value, net	0	(1)
Asset derivatives, Gross asset	0	0
Asset derivatives, Gross liability	0	(1)
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	6
Derivative liabilities net amount on balance sheet	(1)	0
Liability derivatives, Gross asset	0	0
Liability derivatives, Gross liability	(1)	0
Credit default swaps – selling protection | Derivatives not designated as accounting hedging instruments | Other investments
Derivatives, Fair Value
Total asset derivatives, Notional amount	80	80
Total asset derivatives, Fair value, net	1	1
Asset derivatives, Gross asset	1	1
Asset derivatives, Gross liability	0	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	100
Derivative liabilities net amount on balance sheet	0	(3)
Liability derivatives, Gross asset	0	0
Liability derivatives, Gross liability	0	(3)
Other contracts | Derivatives not designated as accounting hedging instruments | Other assets
Derivatives, Fair Value
Total asset derivatives, Notional amount	3	3
Total asset derivatives, Fair value, net	0	1
Asset derivatives, Gross asset	0	1
Asset derivatives, Gross liability	0	0
Options and futures | Derivatives not designated as accounting hedging instruments | Other liabilities & accrued expenses
Derivatives, Fair Value
Total liability derivatives, Notional amount	$ 0	$ 0
Total liability derivatives, Number of contracts | contract	4,464	3,928
Derivative liabilities net amount on balance sheet	$ (53)	$ (37)
Liability derivatives, Gross asset	0	0
Liability derivatives, Gross liability	(53)	(37)
Guaranteed accumulation benefits | Derivatives not designated as accounting hedging instruments | Contractholder funds
Derivatives, Fair Value
Total liability derivatives, Notional amount	225	391
Derivative liabilities net amount on balance sheet	(22)	(34)
Liability derivatives, Gross asset	0	0
Liability derivatives, Gross liability	(22)	(34)
Guaranteed withdrawal benefits | Derivatives not designated as accounting hedging instruments | Contractholder funds
Derivatives, Fair Value
Total liability derivatives, Notional amount	274	290
Derivative liabilities net amount on balance sheet	(12)	(9)
Liability derivatives, Gross asset	0	0
Liability derivatives, Gross liability	(12)	(9)
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	1,735	1,737
Derivative liabilities net amount on balance sheet	(250)	(246)
Liability derivatives, Gross asset	0	0
Liability derivatives, Gross liability	$ (250)	$ (246)