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Summary of Significant Accounting Policies (Self Insurance Retention Liability) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019
Self insurance Retention Liability [Line Items] |  |
Self insurance retention liability | $ 10.0 | $ 3.0
Self insurance retention liability additional deductible |  | 5.0
Self insurance retention Workers' compensation | 2.0 | 1.0
Letters of credit outstanding, amount | 50.9 |
Minimum |  |
Self insurance Retention Liability [Line Items] |  |
Self insurance retention liability additional deductible applicable range |  | 5.0
Maximum |  |
Self insurance Retention Liability [Line Items] |  |
Self insurance retention liability additional deductible applicable range |  | $ 10.0
State of Nebraska [Member] |  |
Self insurance Retention Liability [Line Items] |  |
Workers' compensation insurance bonds | 25.5 |
Workers compensation insurance carrier [Member] |  |
Self insurance Retention Liability [Line Items] |  |
Workers' compensation insurance bonds | 13.4 |
Insurance carrier [Member] |  |
Self insurance Retention Liability [Line Items] |  |
Letters of credit outstanding, amount | $ 39.0 |

X
- Definition The total amount of the contingent obligation under letters of credit outstanding as of the reporting date. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition The disclosure indicates the amount of liability retained by the company (self-insured retention and deductible) for each bodily injury or property damage occurrence. + References No definition available. + Details
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Namespace Prefix: | wern_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The disclosure indicates the amount of additional liability retained by the company (self-insured retention and deductible) for each bodily injury or property damage occurrence between $5.0 million and $10.0 million. + References No definition available. + Details
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Namespace Prefix: | wern_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The disclosure indicates the applicable range for additional liability retained by the company (self-insured retention and deductible) for each bodily injury or property damage occurrence between $5.0 million and $10.0 million. + References No definition available. + Details
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Namespace Prefix: | wern_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The disclosure indicates the amount of liability retained by the company (self-insured retention) for each workers' compensation occurrence. + References No definition available. + Details
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Namespace Prefix: | wern_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition [Line Items] for Self insurance Retention Liability [Table] + References No definition available. + Details
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Namespace Prefix: | wern_
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Balance Type: | na
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- Definition The disclosure indicates the amount of the bonds carried for workers' compensation insurance. + References No definition available. + Details
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Namespace Prefix: | wern_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | wern_EmployeeBenefitsAxis=wern_StateOfNebraskaMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | wern_EmployeeBenefitsAxis=wern_WorkersCompensationInsuranceCarrierMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | wern_EmployeeBenefitsAxis=wern_InsuranceCarriersLettersOfCreditMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |