Investments (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Gain (Loss) on Investments [Line Items]
Non-income producing fixed maturity securities	$ 2,000,000	$ 0
Investments that exceed ten percent of stockholders' equity	0	0
Repayments of senior debt	350,000,000
Limited partnership investments	2,369,000,000	2,371,000,000
Undistributed earnings of limited partnership investments	$ 539,000,000	$ 523,000,000
Percentage of limited partnerships reported on a current basis	61.00%
Percentage of limited partnerships reported on a one month lag	18.00%
Percentage of limited partnerships employing hedge fund strategies focused on fixed income and equity investments	71.00%	68.00%
Limited partnerships invested in private debt and equity	25.00%	27.00%
Percentage of equity related limited partnership hedge fund strategies	62.00%
Percentage of multistrategy approach limited partnership hedge fund strategies	19.00%
Percentage of distressed investments limited partnership hedge fund strategies	17.00%
Percentage of fixed income limited partnership hedge fund strategies	2.00%
Carrying value of ten largest limited partnerships	$ 1,136,000,000	$ 1,168,000,000
Carrying value of limited partnerships as percentage of aggregate partnership equity	3.00%	4.00%
Income As Percentage Of Change In Partnership Equity For All Limited Partnerships	3.00%	4.00%	3.00%
RIght to reclaim cash	$ 0	$ 0
Obligation to return cash	0	0
Derivative, notional amount	167,000,000	174,000,000
Derivative liability, fair value, gross liability	3,000,000	(3,000,000)
Future capital call commitments for limited partnership investments	384,000,000
Mortgage loan commitments	46,000,000
Commitments to purchase or fund privately placed debt securities	165,000,000
Commitments to sell various privately placed debt securities	108,000,000
Carrying value of securities deposited under requirements of regulatory authorities	2,600,000,000	2,300,000,000
Cash and securities deposited as collateral for letters of credit	471,000,000	514,000,000
Carrying value of assets deposited to secure reinsurance and third party obligations	587,000,000	261,000,000
Non-income producing fixed maturity securities	2,000,000	0
Investments that exceed ten percent of stockholders' equity	0	0
Repayments of senior debt	350,000,000
Limited partnership investments	2,369,000,000	2,371,000,000
Undistributed earnings of limited partnership investments	$ 539,000,000	$ 523,000,000
Percentage of limited partnerships reported on a current basis	61.00%
Percentage of limited partnerships reported on a one month lag	18.00%
Percentage of limited partnerships employing hedge fund strategies focused on fixed income and equity investments	71.00%	68.00%
Limited partnerships invested in private debt and equity	25.00%	27.00%
Percentage of equity related limited partnership hedge fund strategies	62.00%
Percentage of multistrategy approach limited partnership hedge fund strategies	19.00%
Percentage of distressed investments limited partnership hedge fund strategies	17.00%
Percentage of fixed income limited partnership hedge fund strategies	2.00%
Carrying value of ten largest limited partnerships	$ 1,136,000,000	$ 1,168,000,000
Carrying value of limited partnerships as percentage of aggregate partnership equity	3.00%	4.00%
Income As Percentage Of Change In Partnership Equity For All Limited Partnerships	3.00%	4.00%	3.00%
RIght to reclaim cash	$ 0	$ 0
Obligation to return cash	0	0
Derivative, notional amount	167,000,000	174,000,000
Derivative liability, fair value, gross liability	(3,000,000)	3,000,000
Future capital call commitments for limited partnership investments	384,000,000
Mortgage loan commitments	46,000,000
Commitments to purchase or fund privately placed debt securities	165,000,000
Commitments to sell various privately placed debt securities	108,000,000
Carrying value of securities deposited under requirements of regulatory authorities	2,600,000,000	2,300,000,000
Cash and securities deposited as collateral for letters of credit	471,000,000	514,000,000
Carrying value of assets deposited to secure reinsurance and third party obligations	587,000,000	$ 261,000,000
Senior Notes, Due November 2019
Gain (Loss) on Investments [Line Items]
Realized investment gains (losses)	42,000,000
Realized investment gains (losses)	$ 42,000,000