| **Benefit Plans**                                     | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
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| **Compensation and Retirement Disclosure [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
| Benefit Plans                                         | Note G. Benefit PlansThe components of net periodic cost (benefit) are presented in the following table.Periods ended September 30Three Months Nine Months(In millions)2015 2014 2015 2014Pension cost (benefit)       Service cost$— $3 $4 $7Interest cost on projected benefit obligation29 33 85 99Expected return on plan assets(44) (48) (131) (144)Amortization of net actuarial loss7 6 26 19Net periodic pension cost (benefit)$(8) $(6) $(16) $(19)        Postretirement cost (benefit)       Interest cost on projected benefit obligation$— $— $— $1Amortization of prior service credit(1) — (2) (9)Amortization of net actuarial loss— — 1 —Curtailment gain— — — (86)Net periodic postretirement cost (benefit)$(1) $— $(1) $(94)In the second quarter of 2015, the Company eliminated future benefit accruals associated with the CNA Retirement Plan (Plan) effective June 30, 2015.  Employees who were continuing to accrue under this Plan up until that date are entitled to an accrued benefit payable based on their eligible compensation and accrued service through June 30, 2015, in accordance with the terms of the Plan.  Starting with the first pay period after July 1, 2015, affected employees began receiving enhanced employer contributions in the CNA 401(k) Plus Plan similar to employees who elected to cease accruals effective December 31, 1999.  Employees who elected to cease accruals effective December 31, 1999 are not affected by this curtailment.  This curtailment resulted in a $55 million decrease in the Plan benefit obligation liability and a reduction of the unrecognized actuarial losses included in AOCI.  In connection with the curtailment, the Company remeasured the plan benefit obligation which resulted in an increase in the discount rate used to determine the benefit obligation from 3.85% to 4.00%.In the second quarter of 2014, the Company eliminated certain postretirement medical benefits associated with the CNA Health and Group Benefits Program.  This change was a negative plan amendment that resulted in an $86 million curtailment gain which was included in Other operating expenses within the Corporate & Other Non-Core segment.  In connection with the plan amendment, the Company remeasured the plan benefit obligation which resulted in a decrease in the discount rate used to determine the benefit obligation from 3.60% to 3.10%. |

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No definition available.
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| - Definition
The entire disclosure for pension and other postretirement benefits.
+ References
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