Derivative Financial Instruments - Summary of the volume and fair value positions of derivative instruments (Details)	Jun. 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	$ 16,000,000
Derivatives, Fair Value
Total asset derivatives, notional amount	640,000,000	$ 255,000,000
Total liability derivatives, notional amount	4,050,000,000	3,133,000,000
Total derivatives, notional amount	$ 4,690,000,000	$ 3,388,000,000
Total asset derivatives, number of contracts | contract	56,028	6,605
Total liability derivatives, number of contracts | contract	61,469	7,128
Total derivatives, Number of contracts | contract	117,497	13,733
Derivative assets net amount on balance sheet	$ 102,000,000	$ 126,000,000
Asset derivatives gross amount	104,000,000	127,000,000
Asset derivatives, gross liability	(2,000,000)	(1,000,000)
Derivative liabilities net amount on balance sheet	(281,000,000)	(363,000,000)
Total derivatives, fair value, net	(179,000,000)	(237,000,000)
Liability derivatives, gross asset	23,000,000	6,000,000
Liability derivatives gross amount	(304,000,000)	(369,000,000)
Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, notional amount		$ 3,114,000,000
Total liability derivatives, number of contracts | contract		7,128
Derivative liabilities net amount on balance sheet		$ (365,000,000)
Liability derivatives, gross asset		4,000,000
Liability derivatives gross amount		(369,000,000)
Foreign currency swap agreements | Other liabilities & accrued expenses | Derivatives designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, notional amount		19,000,000
Derivative liabilities net amount on balance sheet		2,000,000
Liability derivatives, gross asset		2,000,000
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	13,000,000	15,000,000
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	28,000,000	30,000,000
Derivative liabilities net amount on balance sheet	1,000,000	1,000,000
Liability derivatives, gross asset	1,000,000	1,000,000
Liability derivatives gross amount	0	0
Financial 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	718
Total liability derivatives, number of contracts | contract	7,022
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	54,180	6,316
Derivative assets net amount on balance sheet	$ 98,000,000	$ 125,000,000
Asset derivatives gross amount	98,000,000	125,000,000
Asset derivatives, gross liability	0	0
Financial 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	1,130	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	44,000,000
Total liability derivatives, notional amount	7,000,000
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
Total return swap agreements – equity index | Other investments | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, notional amount	147,000,000
Total liability derivatives, notional amount	22,000,000
Derivative assets net amount on balance sheet	1,000,000
Asset derivatives gross amount	1,000,000
Asset derivatives, gross liability	0
Derivative liabilities net amount on balance sheet	0
Liability derivatives, gross asset	0
Liability derivatives gross amount	0
Options and futures | Other liabilities & accrued expenses | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, notional amount	$ 1,032,000,000	$ 0
Total liability derivatives, number of contracts | contract	54,447	7,128
Derivative liabilities net amount on balance sheet	$ (19,000,000)	$ (58,000,000)
Liability derivatives, gross asset	16,000,000	0
Liability derivatives gross amount	(35,000,000)	(58,000,000)
Foreign currency forwards | Other investments | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, notional amount	81,000,000	52,000,000
Derivative assets net amount on balance sheet	0	1,000,000
Asset derivatives gross amount	1,000,000	1,000,000
Asset derivatives, gross liability	(1,000,000)	0
Foreign currency forwards | Other liabilities & accrued expenses | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, notional amount	384,000,000	650,000,000
Derivative liabilities net amount on balance sheet	3,000,000	(17,000,000)
Liability derivatives, gross asset	5,000,000	3,000,000
Liability derivatives gross amount	(2,000,000)	(20,000,000)
Guaranteed accumulation benefits | Contractholder funds | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, notional amount	205,000,000	225,000,000
Derivative liabilities net amount on balance sheet	(19,000,000)	(22,000,000)
Liability derivatives, gross asset	0	0
Liability derivatives gross amount	(19,000,000)	(22,000,000)
Guaranteed withdrawal benefits | Contractholder funds | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, notional amount	252,000,000	274,000,000
Derivative liabilities net amount on balance sheet	(10,000,000)	(12,000,000)
Liability derivatives, gross asset	0	0
Liability derivatives gross amount	(10,000,000)	(12,000,000)
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	1,774,000,000	1,774,000,000
Derivative liabilities net amount on balance sheet	(231,000,000)	(252,000,000)
Liability derivatives, gross asset	0	0
Liability derivatives gross amount	(231,000,000)	(252,000,000)
Credit default swaps – buying protection | Other investments | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, notional amount	102,000,000	105,000,000
Derivative assets net amount on balance sheet	(1,000,000)	(1,000,000)
Asset derivatives gross amount	0	0
Asset derivatives, gross liability	(1,000,000)	(1,000,000)
Credit default swaps – buying protection | Other liabilities & accrued expenses | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total liability derivatives, notional amount	331,000,000	136,000,000
Derivative liabilities net amount on balance sheet	(6,000,000)	(5,000,000)
Liability derivatives, gross asset	1,000,000	0
Liability derivatives gross amount	(7,000,000)	(5,000,000)
Credit default swaps – selling protection | Other investments | Derivatives not designated as accounting hedging instruments
Derivatives, Fair Value
Total asset derivatives, notional amount	250,000,000	80,000,000
Derivative assets net amount on balance sheet	4,000,000	1,000,000
Asset derivatives gross amount	4,000,000	1,000,000
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	15,000,000	25,000,000
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	3,000,000	3,000,000
Derivative assets net amount on balance sheet	0	0
Asset derivatives gross amount	0	0
Asset derivatives, gross liability	$ 0	$ 0