Investments (Narrative) (Details) - USD ($)	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Gain (Loss) on Securities [Line Items]
Non-income producing fixed maturity securities	$ 1,000,000	$ 0
Investments that exceed ten percent of stockholders' equity	0	0
Limited partnership investments	1,752,000,000	1,982,000,000
Undistributed earnings of limited partnership investments	$ 229,000,000	$ 153,000,000
Percentage of limited partnerships reported on a current basis	60.00%
Percentage of limited partnerships reported on a one month lag	10.00%
Percentage of limited partnerships employing hedge fund strategies focused on fixed income and equity investments	61.00%	65.00%
Limited partnerships invested in private debt and equity	33.00%	30.00%
Percentage of equity related limited partnership hedge fund strategies	55.00%
Percentage of multistrategy approach limited partnership hedge fund strategies	18.00%
Percentage of distressed investments limited partnership hedge fund strategies	23.00%
Percentage of fixed income limited partnership hedge fund strategies	4.00%
Carrying value of ten largest limited partnerships	$ 893,000,000	$ 876,000,000
Carrying value of limited partnerships as percentage of aggregate partnership equity	2.00%	2.00%
Income as percentage of change in partnership equity for all limited partnerships	2.00%	3.00%	3.00%
Obligation to return cash	$ 0	$ 0
Notional value	182,000,000	172,000,000
Derivative liability, fair value, gross liability	(7,000,000)	4,000,000
Commitments to purchase or fund privately placed debt securities	945,000,000
Commitments to sell various privately placed debt securities	85,000,000
Carrying value of securities deposited under requirements of regulatory authorities	2,700,000,000	2,500,000,000
Cash and securities deposited as collateral for letters of credit	1,100,000,000	1,000,000,000.0
Senior notes of CNAF, 6.950%, face amount of $150, due November, 2019 \| Senior Notes
Gain (Loss) on Securities [Line Items]
Realized investment gains (losses)	(21,000,000)		$ (42,000,000)
Face amount	500,000,000		$ 350,000,000
Common Stock
Gain (Loss) on Securities [Line Items]
Equity securities, FV-NI, gain (loss)	38,000,000	24,000,000
Preferred stock
Gain (Loss) on Securities [Line Items]
Equity securities, FV-NI, gain (loss)	$ 66,000,000	$ (73,000,000)

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