|**Stock-Based Compensation**                   | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
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|**Share-based Payment Arrangement [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Stock-Based Compensation                       | Stock-Based Compensation The current CNAF Incentive Compensation Plan (the Plan) authorizes the grant of stock-based compensation to certain management personnel for up to11 millionshares of CNAF common stock.  The Plan currently provides for awards of stock options, stock appreciation rights (SARs), restricted shares, restricted stock units (RSUs), performance-based RSUs and performance share units.  The number of shares available for the granting of stock-based compensation under the Plan as ofDecember 31, 2019was approximately2.3 million.In 2016, CNA adopted the Annual Performance Share Plan (PSP).  The PSP provides officers with an opportunity to earn an award based upon attainment of specific performance goals achieved over aone-yearperformance period.  Awards are granted at the beginning of each performance year and are generally subject to atwo-yearcliff vesting period after the Company’s annual performance has been determined.  Prior to the PSP, CNA issued performance share units under the Long Term Incentive Plan (LTI Plan).  The LTI Plan had athree-yearperformance period and was settled during 2018.  In both plans, the performance share units become payable within a range of0%to200%of the number of performance share units initially granted.  Related to the transition to the PSP, CNA granted Special Supplemental Equity Awards (SSE) in 2016, which consisted of restricted stock units that fully vested in 2018.Additionally, the Company may grant RSUs under the Plan in certain circumstances.  These awards generally vest over aonetothree-yearservice period following the grant date.Stock-based compensation that is not fully vested prior to termination is generally forfeited upon termination, except in cases of retirement, death or disability, and as otherwise provided by contractual obligations. The fair value of stock-based compensation awards is based on the market value of the Company's common stock as of the date of grant, except for awards made to foreign participants, which is based on the current market value of the Company’s common stock.  Payments made under the PSP and SSE are made entirely in shares of common stock granted under the Plan, except for awards made to foreign participants, which are paid in cash.The Company recorded stock-based compensation expense related to the Plan of$34 million,$32 millionand$36 millionfor the years endedDecember 31, 2019,2018and2017.  The related income tax benefit recognized was$8 million,$8 millionand$18 millionfor the years endedDecember 31, 2019,2018and2017.  The compensation cost not yet recognized was$38 million, and the weighted average period over which it is expected to be recognized is1.7 yearsas ofDecember 31, 2019.The total fair value of RSUs and performance shares that vested during the years endedDecember 31, 2019,2018and2017was$31 million,$16 millionand$34 million, respectively.The weighted average grant date fair value for RSUs and performance shares granted during the years endedDecember 31, 2019,2018and2017was$43.86,$51.64and$44.20, respectively.The following table presents activity for non-vested RSUs and performance share units under the Plan in2019.Number of AwardsWeighted Average Grant Date Fair ValueBalance as of January 1, 20192,204,148$43.98Awards granted1,051,05343.86Awards vested(801,504)36.81Awards forfeited, canceled or expired(379,425)45.98Performance-based adjustment40,91444.86Balance as of December 31, 20192,115,18646.25
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|                                               | Number of Awards                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Balance as of January 1, 2019                  | 2,204,148                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Awards granted                                 | 1,051,053                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Awards vested                                  | (801,504                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Awards forfeited, canceled or expired          | (379,425                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Performance-based adjustment                   | 40,914                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|Balance as of December 31, 2019                | 2,115,186                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              

X- DefinitionThe entire disclosure for share-based payment arrangement.
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