Derivative Financial Instruments - Summary of the volume and fair value positions of derivative instruments (Details) $ in Millions	Mar. 31, 2018 USD ($) contract	Dec. 31, 2017 USD ($) contract
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Cash and securities pledged in the form of margin deposits	$ 15
Derivatives, Fair Value
Total asset derivatives, notional amount	511	$ 255
Total liability derivatives, notional amount	3,432	3,133
Total derivatives, notional amount	$ 3,943	$ 3,388
Total asset derivatives, number of contracts | contract	7,646	6,605
Total liability derivatives, number of contracts | contract	5,990	7,128
Total derivatives, Number of contracts | contract	13,636	13,733
Derivative assets net amount on balance sheet	$ 94	$ 126
Asset derivatives gross amount	104	127
Asset derivatives, gross liability	(10)	(1)
Derivative liabilities net amount on balance sheet	(313)	(363)
Total derivatives, fair value, net	(219)	(237)
Liability derivatives, gross asset	14	6
Liability derivatives gross amount	(327)	(369)
Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, notional amount	$ 3,413	$ 3,114
Total liability derivatives, number of contracts | contract	5,990	7,128
Derivative liabilities net amount on balance sheet	$ (315)	$ (365)
Liability derivatives, gross asset	12	4
Liability derivatives gross amount	(327)	(369)
Foreign currency swap agreements | Other liabilities & accrued expenses | Derivatives designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, notional amount	19	19
Derivative liabilities net amount on balance sheet	2	2
Liability derivatives, gross asset	2	2
Liability derivatives gross amount	0	0
Interest rate cap agreements | Other investments | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, notional amount	16	15
Derivative assets net amount on balance sheet	0	0
Asset derivatives gross amount	0	0
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	26	30
Derivative liabilities net amount on balance sheet	1	1
Liability derivatives, gross asset	1	1
Liability derivatives gross amount	0	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	5,991	6,316
Derivative assets net amount on balance sheet	$ 102	$ 125
Asset derivatives gross amount	102	125
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	1,655	289
Derivative assets net amount on balance sheet	$ 0	$ 0
Asset derivatives gross amount	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	$ 540	$ 0
Total liability derivatives, number of contracts | contract	5,990	7,128
Derivative liabilities net amount on balance sheet	$ (36)	$ (58)
Liability derivatives, gross asset	10	0
Liability derivatives gross amount	(46)	(58)
Foreign currency forwards | Other investments | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, notional amount	253	52
Derivative assets net amount on balance sheet	(5)	1
Asset derivatives gross amount	2	1
Asset derivatives, gross liability	(7)	0
Foreign currency forwards | Other liabilities & accrued expenses | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, notional amount	316	650
Derivative liabilities net amount on balance sheet	(13)	(17)
Liability derivatives, gross asset	1	3
Liability derivatives gross amount	(14)	(20)
Guaranteed accumulation benefits | Contractholder funds | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, notional amount	215	225
Derivative liabilities net amount on balance sheet	(20)	(22)
Liability derivatives, gross asset	0	0
Liability derivatives gross amount	(20)	(22)
Guaranteed withdrawal benefits | Contractholder funds | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, notional amount	266	274
Derivative liabilities net amount on balance sheet	(10)	(12)
Liability derivatives, gross asset	0	0
Liability derivatives gross amount	(10)	(12)
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	1,770	1,774
Derivative liabilities net amount on balance sheet	(232)	(252)
Liability derivatives, gross asset	0	0
Liability derivatives gross amount	(232)	(252)
Credit default swaps – buying protection | Other investments | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, notional amount	149	105
Derivative assets net amount on balance sheet	(3)	(1)
Asset derivatives gross amount	0	0
Asset derivatives, gross liability	(3)	(1)
Credit default swaps – buying protection | Other liabilities & accrued expenses | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, notional amount	275	136
Derivative liabilities net amount on balance sheet	(5)	(5)
Liability derivatives, gross asset	0	0
Liability derivatives gross amount	(5)	(5)
Credit default swaps – selling protection | Other investments | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, notional amount	90	80
Derivative assets net amount on balance sheet	0	1
Asset derivatives gross amount	0	1
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	5	25
Derivative liabilities net amount on balance sheet	0	0
Liability derivatives, gross asset	0	0
Liability derivatives gross amount	0	0
Other contracts | Other assets | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, notional amount	3	3
Derivative assets net amount on balance sheet	0	0
Asset derivatives gross amount	0	0
Asset derivatives, gross liability	$ 0	$ 0