Derivative Financial Instruments - Summary of the volume and fair value positions of derivative instruments (Details) $ in Millions	Jun. 30, 2019 USD ($) contract	Dec. 31, 2018 USD ($) contract	Jun. 30, 2018 contract
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Cash and securities pledged in the form of margin deposits	$ 46
Derivatives, Fair Value
Total asset derivatives, notional amount	805	$ 470
Total liability derivatives, notional amount	2,630	2,675
Total derivatives, notional amount	$ 3,435	$ 3,145
Total asset derivatives, number of contracts | contract	11,633	13,914
Total liability derivatives, number of contracts | contract	13,265	12,256
Total derivatives, Number of contracts | contract	24,898	26,170
Derivative assets net amount on balance sheet	$ 148	$ 124
Asset derivatives gross amount	152	130
Asset derivatives, gross liability	(4)	(6)
Derivative liabilities net amount on balance sheet	(477)	(269)
Total derivatives, fair value, net	(329)	(145)
Liability derivatives, gross asset	11	12
Liability derivatives gross amount	$ (488)	$ (281)
Derivatives designated as fair value accounting hedging instruments
Derivatives, Fair Value
Total derivatives, Number of contracts | contract	1		1
Other assets | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, number of contracts | contract	561	1,330
Interest rate cap agreements | Other investments | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, notional amount	$ 1	$ 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	35	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	3,050	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	9,362	11,131
Derivative assets net amount on balance sheet	$ 122	$ 115
Asset derivatives gross amount	122	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	1,710	1,453
Derivative assets net amount on balance sheet	$ 2	$ 1
Asset derivatives gross amount	2	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	1,001
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	235	7
Derivative assets net amount on balance sheet	8	0
Asset derivatives gross amount	8	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	95	38
Derivative liabilities net amount on balance sheet	2	(1)
Liability derivatives, gross asset	2	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	172	61
Derivative assets net amount on balance sheet	6	(2)
Asset derivatives gross amount	6	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	9,214	10,956
Derivative liabilities net amount on balance sheet	$ (47)	$ (50)
Liability derivatives, gross asset	0	0
Liability derivatives gross amount	(47)	(50)
Foreign currency forwards | Other investments | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, notional amount	257	258
Derivative assets net amount on balance sheet	14	10
Asset derivatives gross amount	14	11
Asset derivatives, gross liability	0	(1)
Foreign currency forwards | Other liabilities & accrued expenses | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, notional amount	279	341
Derivative liabilities net amount on balance sheet	7	10
Liability derivatives, gross asset	8	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	172	169
Derivative liabilities net amount on balance sheet	(22)	(25)
Liability derivatives, gross asset	0	0
Liability derivatives gross amount	(22)	(25)
Guaranteed withdrawal benefits | Contractholder funds | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, notional amount	209	210
Derivative liabilities net amount on balance sheet	(14)	(14)
Liability derivatives, gross asset	0	0
Liability derivatives gross amount	(14)	(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	(401)	(185)
Liability derivatives, gross asset	0	0
Liability derivatives gross amount	(401)	(185)
Credit default swaps – buying protection | Other investments | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, notional amount	138	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	49	40
Derivative liabilities net amount on balance sheet	(1)	0
Liability derivatives, gross asset	1	0
Liability derivatives gross amount	(2)	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	9	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