Derivative Financial Instruments - Summary of the volume and fair value positions of derivative instruments (Details) $ in Millions	Sep. 30, 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	$ 32
Derivatives, Fair Value
Total asset derivatives, notional amount	859	$ 255
Total liability derivatives, notional amount	2,481	3,133
Total derivatives, notional amount	$ 3,340	$ 3,388
Total asset derivatives, number of contracts | contract	11,872	6,605
Total liability derivatives, number of contracts | contract	6,121	7,128
Total derivatives, Number of contracts | contract	17,993	13,733
Derivative assets net amount on balance sheet	$ 115	$ 126
Asset derivatives gross amount	120	127
Asset derivatives, gross liability	(5)	(1)
Derivative liabilities net amount on balance sheet	(307)	(363)
Total derivatives, fair value, net	(192)	(237)
Liability derivatives, gross asset	1	6
Liability derivatives gross amount	(308)	(369)
Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, notional amount		$ 3,114
Total liability derivatives, number of contracts | contract		7,128
Derivative liabilities net amount on balance sheet		$ (365)
Liability derivatives, gross asset		4
Liability derivatives gross amount		(369)
Foreign currency swap agreements | Other liabilities & accrued expenses | Derivatives designated as accounting hedging instruments
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 amount		0
Interest rate cap agreements | Other investments | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, notional amount	20	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	18	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
Total liability derivatives, notional amount	$ 0
Total asset derivatives, number of contracts | contract	6,813
Total liability derivatives, number of contracts | contract	1,062
Derivative assets net amount on balance sheet	$ 0
Asset derivatives gross amount	0
Asset derivatives, gross liability	0
Derivative liabilities net amount on balance sheet	0
Liability derivatives, gross asset	0
Liability derivatives gross amount	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	3,842	6,316
Derivative assets net amount on balance sheet	$ 106	$ 125
Asset derivatives gross amount	106	125
Asset derivatives, gross liability	0	0
Futures | 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	1,217	289
Derivative assets net amount on balance sheet	$ 0	$ 0
Asset derivatives gross amount	0	0
Asset derivatives, gross liability	0	0
Total return swap agreements – fixed income | Other investments | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, notional amount	60
Total liability derivatives, notional amount	0
Derivative assets net amount on balance sheet	2
Asset derivatives gross amount	2
Asset derivatives, gross liability	0
Derivative liabilities net amount on balance sheet	0
Liability derivatives, gross asset	0
Liability derivatives gross amount	0
Total return swap agreements – equity index | Other investments | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, notional amount	23
Total liability derivatives, notional amount	210
Derivative assets net amount on balance sheet	0
Asset derivatives gross amount	0
Asset derivatives, gross liability	0
Derivative liabilities net amount on balance sheet	(1)
Liability derivatives, gross asset	0
Liability derivatives gross amount	(1)
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	5,059	7,128
Derivative liabilities net amount on balance sheet	$ (40)	$ (58)
Liability derivatives, gross asset	0	0
Liability derivatives gross amount	(40)	(58)
Foreign currency forwards | Other investments | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, notional amount	527	52
Derivative assets net amount on balance sheet	10	1
Asset derivatives gross amount	12	1
Asset derivatives, gross liability	(2)	0
Foreign currency forwards | Other liabilities & accrued expenses | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, notional amount	0	650
Derivative liabilities net amount on balance sheet	0	(17)
Liability derivatives, gross asset	0	3
Liability derivatives gross amount	0	(20)
Guaranteed accumulation benefits | Contractholder funds | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, notional amount	200	225
Derivative liabilities net amount on balance sheet	(17)	(22)
Liability derivatives, gross asset	0	0
Liability derivatives gross amount	(17)	(22)
Guaranteed withdrawal benefits | Contractholder funds | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, notional amount	248	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	1,783	1,774
Derivative liabilities net amount on balance sheet	(239)	(252)
Liability derivatives, gross asset	0	0
Liability derivatives gross amount	(239)	(252)
Credit default swaps – buying protection | Other investments | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, notional amount	222	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	21	136
Derivative liabilities net amount on balance sheet	(1)	(5)
Liability derivatives, gross asset	0	0
Liability derivatives gross amount	(1)	(5)
Credit default swaps – selling protection | Other investments | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, notional amount	4	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	1	25
Derivative liabilities net amount on balance sheet	0	0
Liability derivatives, gross asset	0	0
Liability derivatives gross amount	0	0
Other | 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