Derivative Financial Instruments and Off-balance sheet Financial Instruments - Summary of the volume and fair value positions (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) contract	Dec. 31, 2016 USD ($) contract
Derivative Financial Instruments and Off-balance sheet Financial Instruments
Assumed recoveries under sale of credit protection	$ 0
Cash and securities pledged in the form of margin deposits	10
Derivatives, Fair Value
Total asset derivatives, Notional amount	255	$ 1,103
Total liability derivatives, Notional amount	3,133	2,673
Total derivatives, Notional amount	$ 3,388	$ 3,776
Total asset derivatives, Number of contracts | contract	6,605	4,233
Total liability derivatives, Number of contracts | contract	7,128	4,848
Total derivatives, Number of contracts | contract	13,733	9,081
Net amount on balance sheet	$ 126	$ 93
Asset derivatives, Gross asset	127	121
Asset derivatives, gross liability	(1)	(28)
Net amount on balance sheet	(363)	(334)
Total derivatives, Fair value, net	(237)	(241)
Liability derivatives, Gross asset	6	0
Liability derivatives, Gross liability	(369)	(334)
Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, Notional amount		$ 1,054
Total liability derivatives, Notional amount	$ 3,114
Total asset derivatives, Number of contracts | contract		4,233
Total liability derivatives, Number of contracts | contract	7,128
Net amount on balance sheet		$ 88
Asset derivatives, Gross asset		116
Asset derivatives, gross liability		(28)
Net amount on balance sheet	$ (365)
Liability derivatives, Gross asset	4
Liability derivatives, Gross liability	(369)
Foreign currency swap agreements | Other investments | Derivatives designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, Notional amount		49
Net amount on balance sheet		5
Asset derivatives, Gross asset		5
Asset derivatives, gross liability		0
Foreign currency swap agreements | Other liabilities & accrued expenses | Derivatives designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, Notional amount	19
Net amount on balance sheet	2
Liability derivatives, Gross asset	2
Liability derivatives, Gross liability	0
Interest rate cap agreements | Other investments | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, Notional amount	15	65
Net amount on balance sheet	0	1
Asset derivatives, Gross asset	0	1
Asset derivatives, gross liability	0	0
Interest rate cap agreements | Other liabilities & accrued expenses | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, Notional amount	30
Net amount on balance sheet	1
Liability derivatives, Gross asset	1
Liability derivatives, Gross liability	0
Options | Other investments | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, Notional amount	$ 0	$ 0
Total asset derivatives, Number of contracts | contract	6,316	3,972
Net amount on balance sheet	$ 125	$ 88
Asset derivatives, Gross asset	125	88
Asset derivatives, gross liability	0	0
Financial futures contracts | Other assets | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, Notional amount	$ 0	$ 0
Total asset derivatives, Number of contracts | contract	289	261
Net amount on balance sheet	$ 0	$ 0
Asset derivatives, Gross asset	0	0
Asset derivatives, gross liability	0	0
Options and futures | Other liabilities & accrued expenses | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, Notional amount	$ 0	$ 0
Total liability derivatives, Number of contracts | contract	7,128	4,848
Net amount on balance sheet	$ (58)	$ (39)
Liability derivatives, Gross asset	0	0
Liability derivatives, Gross liability	(58)	(39)
Foreign currency forwards | Other investments | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, Notional amount	52	759
Net amount on balance sheet	1	0
Asset derivatives, Gross asset	1	24
Asset derivatives, gross liability	0	(24)
Foreign currency forwards | Other liabilities & accrued expenses | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, Notional amount	650
Net amount on balance sheet	(17)
Liability derivatives, Gross asset	3
Liability derivatives, Gross liability	(20)
Guaranteed accumulation benefits | Contractholder funds | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, Notional amount	225	391
Net amount on balance sheet	(22)	(34)
Liability derivatives, Gross asset	0	0
Liability derivatives, Gross liability	(22)	(34)
Guaranteed withdrawal benefits | Contractholder funds | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, Notional amount	274	290
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 | Contractholder funds | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, Notional amount	1,774	1,751
Net amount on balance sheet	(252)	(247)
Liability derivatives, Gross asset	0	0
Liability derivatives, Gross liability	(252)	(247)
Credit default swaps – buying protection | Other investments | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, Notional amount	105	87
Net amount on balance sheet	(1)	(4)
Asset derivatives, Gross asset	0	0
Asset derivatives, gross liability	(1)	(4)
Credit default swaps – buying protection | Other liabilities & accrued expenses | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, Notional amount	136	136
Net amount on balance sheet	(5)	(2)
Liability derivatives, Gross asset	0	0
Liability derivatives, Gross liability	(5)	(2)
Credit default swaps – selling protection | Other investments | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, Notional amount	80	140
Net amount on balance sheet	1	2
Asset derivatives, Gross asset	1	2
Asset derivatives, gross liability	0	0
Credit default swaps – selling protection | Other liabilities & accrued expenses | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, Notional amount	25	105
Net amount on balance sheet	0	(3)
Liability derivatives, Gross asset	0	0
Liability derivatives, Gross liability	0	(3)
Other contracts | Other assets | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, Notional amount	3	3
Net amount on balance sheet	0	1
Asset derivatives, Gross asset	0	1
Asset derivatives, gross liability	$ 0	$ 0