|**Stock-Based Compensation**                                                  | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
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|**Disclosure of Compensation Related Costs, Share-based Payments [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Stock-Based Compensation                                                      | Stock-Based CompensationThe CNAF Incentive Compensation Plan (the Plan), as amended and restated on January 1, 2010, as amended, 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, 2017was approximately2.8 million.The Company recorded stock-based compensation expense related to the Plan of$36 million,$36 millionand$14 millionfor the years endedDecember 31, 2017,2016and2015.  The related income tax benefit recognized was$18 million,$12 millionand$5 millionfor the years endedDecember 31, 2017,2016, and2015.  The compensation cost not yet recognized was$41 million, and the weighted average period over which it is expected to be recognized is1.8years as ofDecember 31, 2017.Equity 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.Share AwardsThe fair value of share awards is based on the market value of the Company's common stock as of the date of grant.  Share awards currently granted under the Plan include RSUs, performance-based RSUs and performance share units.  Generally, RSU's vest over atwoorthree-year service period following the date of grant.  Performance-based RSUs generally become payable within a range of0%to100%of the number of shares initially granted based upon the attainment of specific annual performance goals and vest ratably over afour-year service period following the date of grant.  As ofDecember 31, 2017the Company does not have any outstanding performance-based RSUs.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-year performance period.  Awards are granted at the beginning of each performance year and are generally subject to atwo-year cliff 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-year performance period and continued to have outstanding awards through 2017.  In both plans, the performance share units become payable within a range of0%to200%of the number of performance share units initially granted.In 2016, CNA granted Special Supplemental Equity Awards (SSE).  The awards consist of restricted stock units subject to both Company performance in 2016 and service-based vesting up totwoyears.  Payments made under the PSP and SSE are made entirely in shares of common stock, except for awards made to foreign participants, which are paid in cash.The following table presents activity for non-vested RSUs, performance-based RSUs and performance share units under the Plan in2017.Number of AwardsWeighted-Average Grant Date Fair ValueBalance as of January 1, 20172,131,782$33.28Awards granted1,010,74144.20Awards vested(930,616)33.31Awards forfeited, canceled or expired(290,296)36.20Performance-based adjustment149,12644.07Balance as of December 31, 20172,070,737$38.92
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|                                                                              | Number of Awards                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Balance as of January 1, 2017                                                 | 2,131,782                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Awards granted                                                                | 1,010,741                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Awards vested                                                                 | (930,616                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Awards forfeited, canceled or expired                                         | (290,296                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Performance-based adjustment                                                  | 149,126                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Balance as of December 31, 2017                                               | 2,070,737                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      

X- DefinitionThe entire disclosure for compensation-related costs for equity-based compensation, which may include disclosure of policies, compensation plan details, allocation of equity compensation, incentive distributions, equity-based arrangements to obtain goods and services, deferred compensation arrangements, employee stock ownership plan details and employee stock purchase plan details.
+ ReferencesReference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -URI http://asc.fasb.org/topic&trid=2228938Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 50 -URI http://asc.fasb.org/subtopic&trid=2208855
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