Derivative Financial Instruments - Summary of the volume and fair value positions of derivative instruments (Details) $ in Millions	Sep. 30, 2019 USD ($) contract	Dec. 31, 2018 USD ($) contract	Sep. 30, 2018 contract
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Cash and securities pledged in the form of margin deposits	$ 50
Derivatives, Fair Value
Total asset derivatives, notional amount	645	$ 470
Total liability derivatives, notional amount	2,836	2,675
Total derivatives, notional amount	$ 3,481	$ 3,145
Total asset derivatives, number of contracts | contract	13,571	13,914
Total liability derivatives, number of contracts | contract	13,314	12,256
Total derivatives, Number of contracts | contract	26,885	26,170
Derivative assets net amount on balance sheet	$ 150	$ 124
Asset derivatives gross amount	155	130
Asset derivatives, gross liability	(5)	(6)
Derivative liabilities net amount on balance sheet	(477)	(269)
Total derivatives, fair value, net	(327)	(145)
Liability derivatives, gross asset	32	12
Liability derivatives gross amount	(509)	$ (281)
Derivatives designated as fair value accounting hedging instruments
Derivatives, Fair Value
Total derivatives, Number of contracts | contract			1
Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, notional amount	$ 643
Total asset derivatives, number of contracts | contract	13,571
Derivative assets net amount on balance sheet	$ 150
Asset derivatives gross amount	155
Asset derivatives, gross liability	$ (5)
Other assets | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, number of contracts | contract	2,674	1,330
Interest rate cap agreements | Other investments | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, notional amount	$ 21	$ 6
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	14	31
Derivative liabilities net amount on balance sheet	0	1
Liability derivatives, gross asset	0	1
Liability derivatives gross amount	0	0
Futures | Other assets | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, notional amount	0	0
Derivative assets net amount on balance sheet	0	1
Asset derivatives gross amount	0	1
Asset derivatives, gross liability	0	0
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	2,915	1,300
Derivative liabilities net amount on balance sheet	$ 0	$ (1)
Liability derivatives, gross asset	0	0
Liability derivatives gross amount	0	(1)
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	9,290	11,131
Derivative assets net amount on balance sheet	$ 131	$ 115
Asset derivatives gross amount	131	115
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	1,607	1,453
Derivative assets net amount on balance sheet	$ 1	$ 1
Asset derivatives gross amount	1	1
Asset derivatives, gross liability	0	0
Futures | Other liabilities & accrued expenses | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, notional amount	$ 0
Total liability derivatives, number of contracts | contract	1,383
Derivative liabilities net amount on balance sheet	$ (1)
Liability derivatives, gross asset	0
Liability derivatives gross amount	(1)
Total return swap agreements – fixed income | Other investments | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, notional amount	7	7
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 liabilities & accrued expenses | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, notional amount	228	38
Derivative liabilities net amount on balance sheet	8	(1)
Liability derivatives, gross asset	8	0
Liability derivatives gross amount	0	(1)
Total return swap agreements – equity index | Other investments | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, notional amount	170	61
Derivative assets net amount on balance sheet	3	(2)
Asset derivatives gross amount	3	0
Asset derivatives, gross liability	0	(2)
Total return swap agreements – equity index | Other liabilities & accrued expenses | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, notional amount		71
Derivative liabilities net amount on balance sheet		(4)
Liability derivatives, gross asset		0
Liability derivatives gross amount		(4)
Options | 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	9,016	10,956
Derivative liabilities net amount on balance sheet	$ (52)	$ (50)
Liability derivatives, gross asset	0	0
Liability derivatives gross amount	(52)	(50)
Foreign currency forwards | Other investments | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, notional amount	291	258
Derivative assets net amount on balance sheet	19	10
Asset derivatives gross amount	20	11
Asset derivatives, gross liability	(1)	(1)
Foreign currency forwards | Other liabilities & accrued expenses | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, notional amount	396	341
Derivative liabilities net amount on balance sheet	23	10
Liability derivatives, gross asset	24	11
Liability derivatives gross amount	(1)	(1)
Guaranteed accumulation benefits | Contractholder funds | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, notional amount	161	169
Derivative liabilities net amount on balance sheet	(21)	(25)
Liability derivatives, gross asset	0	0
Liability derivatives gross amount	(21)	(25)
Guaranteed withdrawal benefits | Contractholder funds | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, notional amount	205	210
Derivative liabilities net amount on balance sheet	(16)	(14)
Liability derivatives, gross asset	0	0
Liability derivatives gross amount	(16)	(14)
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	1,782	1,770
Derivative liabilities net amount on balance sheet	(417)	(185)
Liability derivatives, gross asset	0	0
Liability derivatives gross amount	(417)	(185)
Credit default swaps – buying protection | Other investments | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, notional amount	149	136
Derivative assets net amount on balance sheet	(4)	(1)
Asset derivatives gross amount	0	2
Asset derivatives, gross liability	(4)	(3)
Credit default swaps – buying protection | Other liabilities & accrued expenses | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, notional amount	46	40
Derivative liabilities net amount on balance sheet	(1)	0
Liability derivatives, gross asset	0	0
Liability derivatives gross amount	(1)	0
Credit default swaps – selling protection | Other investments | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, notional amount	5
Derivative assets net amount on balance sheet	0
Asset derivatives gross amount	0
Asset derivatives, gross liability	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	4	5
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 designated as fair value accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, notional amount	2
Derivative assets net amount on balance sheet	0
Asset derivatives gross amount	0
Asset derivatives, gross liability	$ 0
Other | Other assets | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, notional amount		2
Derivative assets net amount on balance sheet		0
Asset derivatives gross amount		0
Asset derivatives, gross liability		$ 0