| **Benefit Plans (Tables)**                                                                                                                          | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| **Compensation and Retirement Disclosure [Abstract]**                                                                                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
| Funded status                                                                                                                                       | The following table presents a reconciliation of benefit obligations and plan assets. Pension Benefits Postretirement Benefits(In millions)2015 2014 2015 2014Benefit obligation as of January 1$3,019 $2,943 $29 $40Changes in benefit obligation:       Service cost4 9 — —Interest cost112 132 1 1Participants' contributions— — 4 5Plan amendments(55) (3) — (7)Actuarial (gain) loss(79) 367 (3) 1Benefits paid(173) (165) (8) (11)Settlements— (257) — —Foreign currency translation and other(7) (7) — —Benefit obligation as of December 312,821 3,0192329Fair value of plan assets as of January 12,456 2,656 — —Change in plan assets:       Actual return on plan assets(18) 216 — —Company contributions10 12 4 6Participants' contributions— — 4 5Benefits paid(173) (165) (8) (11)Settlements— (257) — —Foreign currency translation and other(8) (6) — —Fair value of plan assets as of December 312,267 2,456 — —Funded status$(554) $(563) $(23) $(29)Amounts recognized on the Consolidated Balance Sheets as of December 31:       Other assets$12 $9 $— $—Other liabilities(566) (572) (23) (29)Net amount recognized$(554) $(563) $(23) $(29)Amounts recognized in Accumulated other comprehensive income, not yet recognized in net periodic cost (benefit):       Prior service credit$— $— $(6) $(9)Net actuarial loss999 974 3 8Net amount recognized$999 $974 $(3) $(1)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
| Components of net periodic cost (benefit)                                                                                                           | The components of net periodic cost (benefit) are presented in the following table.Years ended December 31     (In millions)2015 2014 2013Pension cost (benefit)     Service cost$4 $9 $12Interest cost on projected benefit obligation112 132 121Expected return on plan assets(174) (191) (181)Amortization of net actuarial loss34 25 47Settlement loss— 84 3Net periodic pension cost (benefit)$(24) $59 $2      Postretirement cost (benefit)     Service cost$— $— $1Interest cost on projected benefit obligation1 1 1Amortization of prior service credit(3) (10) (18)Amortization of net actuarial loss1 1 2Curtailment gain—(86)—Net periodic postretirement cost (benefit)$(1) $(94) $(14)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
| Schedule of amounts recognized in Other comprehensive income                                                                                        | The amounts recognized in Other comprehensive income are presented in the following table.Years ended December 31     (In millions)2015 2014 2013Pension and postretirement benefits     Amounts arising during the period$(111) $(337) $422Curtailment and other56 (81) —Settlement— 84 —Reclassification adjustment relating to prior service credit(3) (10) (18)Reclassification adjustment relating to actuarial loss35 26 49Total increase (decrease) in Other comprehensive income$(23) $(318) $453                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
| Schedule of estimated amounts to be recognized from Accumulated other comprehensive income into net periodic cost (benefit) during next fiscal year | The table below presents the estimated amounts to be recognized from AOCI into net periodic cost (benefit) during 2016.(In millions) PensionBenefits Postretirement BenefitsAmortization of prior service credit $— $(2)Amortization of net actuarial loss 37 —Total estimated amounts to be recognized $37 $(2)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
| Actuarial assumptions for benefit obligations and for net cost or benefit                                                                           | Actuarial assumptions used for the CNA Retirement Plan and CNA Health and Group Benefits Program to determine benefit obligations are presented in the following table.December 312015 2014Pension benefits   Discount rate4.150% 3.850%Expected long term rate of return7.500 7.500Rate of compensation increasesN/A 3.920Postretirement benefits   Discount rate2.750% 2.500%Actuarial assumptions used for the CNA Retirement Plan and CNA Health and Group Benefits Program to determine net cost or benefit are presented in the following table.Years ended December 312015 2014 2013Pension benefits     Discount rate3.850%/4.000% 4.650% 3.800%Expected long term rate of return7.500 7.500 7.750Rate of compensation increases3.920 3.990 4.066Postretirement benefits     Discount rate2.500% 3.600%/3.100% 2.800%                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
| Fair value of plan assets measured on a recurring basis                                                                                             | Pension plan assets measured at fair value on a recurring basis as well as cash are presented in the following tables. December 31, 2015        (In millions) Level 1 Level 2 Level 3 Total Assets at Fair ValueAssets        Fixed maturity securities:        Corporate and other bonds $— $455 $10 $465States, municipalities and political subdivisions — 106 — 106Asset-backed:        Residential mortgage-backed — 133 — 133Commercial mortgage-backed — 69 — 69Other asset-backed — 11 — 11Total asset-backed — 213 — 213U.S. Treasury and obligations of government-sponsored enterprises — — — —Total fixed maturity securities — 774 10 784Equity securities 336 107 — 443Derivative financial instruments 1 — — 1Short term investments 24 28 — 52Limited partnerships:        Hedge funds — 516 296 812Private equity — — 123 123Total limited partnerships — 516 419 935Other assets — 52 — 52Total assets $361 $1,477 $429 $2,267December 31, 2014        (In millions) Level 1 Level 2 Level 3 Total Assets at Fair ValueAssets        Fixed maturity securities:        Corporate and other bonds $— $463 $15 $478States, municipalities and political subdivisions — 80 — 80Asset-backed:        Residential mortgage-backed — 123 — 123Commercial mortgage-backed — 75 — 75   Other asset-backed — 12 — 12Total asset-backed — 210 — 210U.S. Treasury and obligations of government-sponsored enterprises 25 — — 25Total fixed maturity securities 25 753 15 793Equity securities 389 118 — 507Derivative financial instruments 1 — — 1Short term investments 33 101 — 134Limited partnerships:       Hedge funds — 562 303 865Private equity — — 113 113Total limited partnerships — 562 416 978Other assets — 30 — 30Cash 13 — — 13Total assets $461 $1,564 $431 $2,456 |
| Reconciliation of level 3 plan assets                                                                                                               | The tables below present a reconciliation for all pension plan assets measured at fair value on a recurring basis using significant unobservable inputs (Level 3).Level 3(In millions)Balance as of January 1, 2015 Actual return on assets still held as of December 31, 2015 Actual return on assets sold during the year ended December 31, 2015 Purchases, sales and settlements Net transfers into (out of) Level 3 Balance as of December 31, 2015Fixed maturity securities:           Corporate and other bonds$15 $— $— $— $(5) $10Limited partnerships:           Hedge funds303 18 — (25) — 296Private equity113 9 — 1 — 123Total limited partnerships416 27 — (24) — 419Total$431 $27 $— $(24) $(5) $429Level 3(In millions)Balance as of January 1, 2014 Actual return on assets still held as of December 31, 2014 Actual return on assets sold during the year ended December 31, 2014 Purchases, sales and settlements Net transfers into (out of) Level 3 Balance as of December 31, 2014Fixed maturity securities:           Corporate and other bonds$15 $— $— $— $— $15Equity securities8 — — (8) — —Limited partnerships:           Hedge funds322 19 — (38) — 303Private equity114 19 — (20) — 113Total limited partnerships436 38 — (58) — 416Total$459 $38 $— $(66) $— $431                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
| Estimated future minimum benefit payments to participants                                                                                           | The table below presents the estimated future minimum benefit payments to participants as of December 31, 2015.(In millions)Pension Benefits Postretirement Benefits2016$180 $42017183 42018184 32019183 32020185 22021-2025915 7                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |

| X                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_CompensationAndRetirementDisclosureAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tabular disclosure of the major categories of plan assets of pension plans and/or other employee benefit plans.  This information may include, but is not limited to, the target allocation of plan assets, the fair value of each major category of plan assets, and the level within the fair value hierarchy in which the fair value measurements fall.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(5) -URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920
+ Details**Name:**us-gaap_ScheduleOfAllocationOfPlanAssetsTableTextBlock**Namespace Prefix:**us-gaap_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tabular disclosure of the amounts related to pension plans and/or other employee benefit plans in accumulated other comprehensive income (loss) expected to be recognized as components of net periodic benefit cost over the next fiscal year, including the net gain (loss), net prior service cost or credit, and net transition asset or obligation.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (s) -URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920
+ Details**Name:**us-gaap_ScheduleOfAmountsInAccumulatedOtherComprehensiveIncomeLossToBeRecognizedOverNextFiscalYearTableTextBlock**Namespace Prefix:**us-gaap_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tabular disclosure of the net gain (loss) and net prior service cost or credit recognized in other comprehensive income (loss) for the period for pension plans and/or other employee benefit plans, and reclassification adjustments of other comprehensive income (loss) for the period, as those amounts, including amortization of the net transition asset or obligation, are recognized as components of net periodic benefit cost.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920
+ Details**Name:**us-gaap_ScheduleOfAmountsRecognizedInOtherComprehensiveIncomeLossTableTextBlock**Namespace Prefix:**us-gaap_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tabular disclosure of the assumptions used to determine for pension plans and/or other employee benefit plans the benefit obligation and net benefit cost, including assumed discount rates, rate increase in compensation increase, and expected long-term rates of return on plan assets.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k) -URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920
+ Details**Name:**us-gaap_ScheduleOfAssumptionsUsedTableTextBlock**Namespace Prefix:**us-gaap_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tabular disclosure of the effect of fair value measurements using significant unobservable inputs (Level 3) on changes in plan assets of pension plans and/or other employee benefit plans for the period.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(4) -URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920
+ Details**Name:**us-gaap_ScheduleOfEffectOfSignificantUnobservableInputsChangesInPlanAssetsTableTextBlock**Namespace Prefix:**us-gaap_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tabular disclosure of benefits expected to be paid by pension plans and/or other employee benefit plans in each of the next five fiscal years and in the aggregate for the five fiscal years thereafter.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920
+ Details**Name:**us-gaap_ScheduleOfExpectedBenefitPaymentsTableTextBlock**Namespace Prefix:**us-gaap_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tabular disclosure of the components of net benefit costs for pension plans and/or other employee benefit plans including service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) recognized due to settlements or curtailments.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h) -URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920
+ Details**Name:**us-gaap_ScheduleOfNetBenefitCostsTableTextBlock**Namespace Prefix:**us-gaap_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tabular disclosure of net funded status of pension plans and/or other employee benefit plans.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920
+ Details**Name:**us-gaap_ScheduleOfNetFundedStatusTableTextBlock**Namespace Prefix:**us-gaap_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |