| **Benefit Plans (Narrative) (details) (USD $)In Millions, unless otherwise specified**     | 12 Months Ended                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| Lump sum settlement acceptance rate by retirement plan participants                        | 20.00%cna_Lumpsumsettlementacceptanceratebyretirementplanparticipants                                                                                                                                        |   |   |
| Lump sum pension settlement payment                                                        | $ 253cna_Lumpsumpensionsettlementpayment                                                                                                                                                                     |   |   |
| Settlement loss                                                                            | 84cna_DefinedBenefitPlanRecognizedNetLossDuetoSettlements                                                                                                                                                    |   |   |
| Curtailment gain                                                                           | (86)cna_DefinedBenefitPlanRecognizedNetGainDuetoCurtailments                                                                                                                                                 |   |   |
| Future capital call commitments for limited partnership investments                        | 327cna_FutureCapitalCallCommitments                                                                                                                                                                          |   |   |
| Document Period End Date                                                                   | Dec. 31,  2014                                                                                                                                                                                               |   |   |
| Pension Plans, Defined Benefit [Member]                                                    |                                                                                                                                                                                                         |   |   |
| Settlement loss                                                                            | 84cna_DefinedBenefitPlanRecognizedNetLossDuetoSettlements/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                                           | 3cna_DefinedBenefitPlanRecognizedNetLossDuetoSettlements/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember |   |
| Defined benefit plan, accumulated benefit obligation                                       | 2,962us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                                         | 2,889us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember |   |
| Future capital call commitments for limited partnership investments                        | 107cna_FutureCapitalCallCommitments/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                                                                 |   |   |
| Limited partnership equity related hedge fund strategy plan assets                         | 57.00%cna_LimitedPartnershipEquityRelatedHedgeFundStrategyPlanAssets/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                                |   |   |
| Limited partnership hedge fund multi strategy plan assets                                  | 37.00%cna_LimitedPartnershipHedgeFundMultiStrategyPlanAssets/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                                        |   |   |
| Limited partnership hedge fund strategy distressed investments plan assets                 | 6.00%cna_LimitedPartnershipHedgeFundStrategyDistressedInvestmentsPlanAssets/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                         |   |   |
| Defined Benefit Plans, Estimated Future Employer Contributions in Next Fiscal Year         | 14us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInNextFiscalYear/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                   |   |   |
| Cost of Living Adjustment [Member]                                                         |                                                                                                                                                                                                         |   |   |
| Assumptions used in calculating assumed health care cost trend rate                        | 4.00%cna_Assumptionsusedincalculatingassumedhealthcarecosttrendrate/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= cna_CostofLivingAdjustmentMember                                         |   |   |
| Employer Subsidy on Health Care Costs [Member]                                             |                                                                                                                                                                                                         |   |   |
| Assumptions used in calculating assumed health care cost trend rate                        | 4.00%cna_Assumptionsusedincalculatingassumedhealthcarecosttrendrate/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= cna_EmployerSubsidyonHealthCareCostsMember                               |   |   |
| Other Postretirement Benefit Plan [Member]                                                 |                                                                                                                                                                                                         |   |   |
| Curtailment gain                                                                           | (86)cna_DefinedBenefitPlanRecognizedNetGainDuetoCurtailments/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember                     |   |   |
| Defined benefit plan, health care cost trend rate assumed for next fiscal year             | 4.00%us-gaap_DefinedBenefitPlanHealthCareCostTrendRateAssumedForNextFiscalYear/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember   | 4.00%us-gaap_DefinedBenefitPlanHealthCareCostTrendRateAssumedForNextFiscalYear/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember | 4.00%us-gaap_DefinedBenefitPlanHealthCareCostTrendRateAssumedForNextFiscalYear/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember |
| Defined Benefit Plans, Estimated Future Employer Contributions in Next Fiscal Year         | 5us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInNextFiscalYear/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember |   |   |
| Equity securities [Member]                                                                 |                                                                                                                                                                                                         |   |   |
| Defined Benefit Plan, Target Plan Asset Allocations Range Minimum                          | 40.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMinimum/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_EquitySecuritiesMember                                        |   |   |
| Defined Benefit Plan, Target Plan Asset Allocations Range Maximum                          | 60.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMaximum/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_EquitySecuritiesMember                                        |   |   |
| Defined Contribution Pension [Member]                                                      |                                                                                                                                                                                                         |   |   |
| Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent              | 50.00%us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPensionPlansDefinedBenefitMember              |   |   |
| Defined Contribution Plan, Employer Matching Contribution, Percent of Match                | 70.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercentOfMatch/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPensionPlansDefinedBenefitMember                |   |   |
| Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay | 6.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPensionPlansDefinedBenefitMember                        |   |   |
| Defined Contribution Plan, Employers Matching Contribution, Vesting Period in Years        | 5 years                                                                                                                                                                                                      |   |   |
| Company Contribution of Eligible Compensation, Depending on Age, Percent                   | 3% or 5%                                                                                                                                                                                                     |   |   |
| Benefit expense for the Company's savings plan                                             | $ 69us-gaap_DefinedContributionPlanCostRecognized/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPensionPlansDefinedBenefitMember                                              | 71us-gaap_DefinedContributionPlanCostRecognized/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPensionPlansDefinedBenefitMember | 70us-gaap_DefinedContributionPlanCostRecognized/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_OtherPensionPlansDefinedBenefitMember |
| First Year of Employment [Member]                                                          |                                                                                                                                                                                                         |   |   |
| Defined Contribution Plan, Employer Matching Contribution, Percent of Match                | 35.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercentOfMatch/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= cna_FirstYearofEmploymentMember                              |   |   |
| Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay | 6.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= cna_FirstYearofEmploymentMember                                      |   |   |
| Additional Contribution, Management Discretion [Member]                                    |                                                                                                                                                                                                         |   |   |
| Defined Contribution Plan, Employer Matching Contribution, Percent of Match                | 80.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercentOfMatch/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= cna_AdditionalContributionManagementDiscretionMember         |   |   |
| Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay | 6.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= cna_AdditionalContributionManagementDiscretionMember                 |   |   |
| Defined Contribution Plan, Employers Matching Contribution, Vesting Period in Years        | 5 years                                                                                                                                                                                                      |   |   |
| Defined Contribution Plan, Employer Discretionary Contribution Amount, Percent             | 2.00%cna_DefinedContributionPlanEmployerDiscretionaryContributionAmountPercent/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= cna_AdditionalContributionManagementDiscretionMember          |   |   |

| X                                                                                                                                                                                                                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Assumptions used in calculating assumed health care cost trend rate, including cost-of-living adjustment and employer subsidy on health care costs
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Company Contribution of Eligible Compensation, Depending on Age, Percent
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of net gain recognized in net periodic benefit cost as a result of an event that significantly reduces the expected years of future service of present employees or eliminates for a significant number of employees the accrual of defined benefits for some or all of their future services.
+ References
No definition available.
+ Details**Name:**cna_DefinedBenefitPlanRecognizedNetGainDuetoCurtailments**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of net loss recognized in net periodic benefit cost as a result of an irrevocable action that relieves the employer (or the plan) of primary responsibility for a benefit obligation and eliminates significant risks related to the obligation and the assets used to effect the settlement. Examples of transactions that constitute a settlement include, but are not limited to, lump-sum cash payments to plan participants in exchange for their rights to receive specified benefits and purchasing nonparticipating annuity contracts to cover vested benefits.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Defined Contribution Plan, Employer Discretionary Contribution Amount, Percent
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Defined Contribution Plan, Employers Matching Contribution, Vesting Period in Years
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Future capital call commitments to limited partnerships for which we would receive an ownership interest.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of equity related hedge fund strategies within limited partnership plan assets
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of limited partnership plan assets that pursue a multi-strategy approach
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of limited partnership plan assets that pursue distressed investments
+ References
No definition available.
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| X                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Lump sum pension settlement payment
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Lump sum settlement acceptance rate by retirement plan participants
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The end date of the period reflected on the cover page if a periodic report. For all other reports and registration statements containing historical data, it is the date up through which that historical data is presented.  If there is no historical data in the report, use the filing date. The format of the date is CCYY-MM-DD.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
For defined benefit pension plans, the actuarial present value of benefits (whether vested or nonvested) attributed by the pension benefit formula to employee service rendered before a specified date and based on employee service and compensation (if applicable) before that date. The accumulated benefit obligation differs from the projected benefit obligation in that it includes no assumption about future compensation levels. For plans with flat-benefit or nonpay-related pension benefit formulas, the accumulated benefit obligation and the projected benefit obligation are the same.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The assumed health care cost trend rate for the next year used to measure the expected cost of benefits covered by the plan (gross eligible charges). This is based upon the annual rate of change in the cost of health care benefits currently provided by the postretirement benefit plan, due to factors other than changes in the composition of the plan population by age and dependency status.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of employer's best estimate of contributions expected to be paid to the plan during the next fiscal year beginning after the date of the latest statement of financial position.
+ 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 (g) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Target allocation maximum percentage of investments of this type to total plan assets presented on a weighted average basis.
+ 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)(i) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 5 -Subparagraph (c)(i) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e2446-114920
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Target allocation minimum percentage of investments of this type to total plan assets presented on a weighted average basis.
+ 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)(i) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 5 -Subparagraph (c)(i) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e2446-114920
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of the cost recognized during the period for defined contribution plans.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage employer matches of the employee's percentage contribution matched.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan.
+ References
No definition available.
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| X                                                                                                                                                                                                          |
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| X                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                                 |
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