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Credit Facilities (Narrative) (Details) (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Line of Credit Facility [Line Items] |  |  |
Total of committed credit facilities with two banks | $ 325,000,000 us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity |  |
Borrowings outstanding | 75,000,000 us-gaap_LineOfCredit | 40,000,000 us-gaap_LineOfCredit |
Standby letters of credit | 32,700,000 us-gaap_LettersOfCreditOutstandingAmount |  |
Additional borrowings of credit facilities | 75,000,000 us-gaap_ProceedsFromLongTermLinesOfCredit | 10,000,000 us-gaap_ProceedsFromLongTermLinesOfCredit | 70,000,000 us-gaap_ProceedsFromLongTermLinesOfCredit
Credit facility repayments - long-term | 40,000,000 wern_Creditfacilityrepaymentslongterm |  |
Mature On May 31, 2016 |  |  |
Line of Credit Facility [Line Items] |  |  |
Total of committed credit facilities with two banks | 175,000,000 us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_LineOfCreditFacilityAxis = wern_LineOfCreditFacilityMatureDateAtMayThirtyOneTwoThousandAndSixteenMember |  |
Committed credit facilities maturity | May 31, 2016 |  |
Number of financial covenants for debt agreements | 2 wern_LineOfCreditFacilityNumberOfCovenants / us-gaap_LineOfCreditFacilityAxis = wern_LineOfCreditFacilityMatureDateAtMayThirtyOneTwoThousandAndSixteenMember |  |
Mature On May 31, 2017 |  |  |
Line of Credit Facility [Line Items] |  |  |
Total of committed credit facilities with two banks | 75,000,000 us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_LineOfCreditFacilityAxis = wern_LineOfCreditFacilityMatureDateAtMayThirtyOneTwoThousandAndSeventeenMember |  |
Committed credit facilities maturity | May 31, 2017 |  |
Number of financial covenants for debt agreements | 2 wern_LineOfCreditFacilityNumberOfCovenants / us-gaap_LineOfCreditFacilityAxis = wern_LineOfCreditFacilityMatureDateAtMayThirtyOneTwoThousandAndSeventeenMember |  |
Mature On September 15, 2019 |  |  |
Line of Credit Facility [Line Items] |  |  |
Total of committed credit facilities with two banks | $ 75,000,000 us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_LineOfCreditFacilityAxis = wern_LineofCreditFacilityMatureDateatSeptemberFifteenTwoThousandandNineteenMemberDomain |  |
Committed credit facilities maturity | Sep. 15, 2019 |  |
Line of credit facility interest rate | 0.76%us-gaap_LineOfCreditFacilityInterestRateAtPeriodEnd / us-gaap_LineOfCreditFacilityAxis = wern_LineofCreditFacilityMatureDateatSeptemberFifteenTwoThousandandNineteenMemberDomain |  |
Interest Rate Swap Facility, Fixed Interest | 2.50%us-gaap_DerivativeFixedInterestRate / us-gaap_LineOfCreditFacilityAxis = wern_LineofCreditFacilityMatureDateatSeptemberFifteenTwoThousandandNineteenMemberDomain |  |
Number of financial covenants for debt agreements | 2 wern_LineOfCreditFacilityNumberOfCovenants / us-gaap_LineOfCreditFacilityAxis = wern_LineofCreditFacilityMatureDateatSeptemberFifteenTwoThousandandNineteenMemberDomain |  |

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- Definition Fixed interest rate related to the interest rate derivative. + References No definition available. + Details
Name: | us-gaap_DerivativeFixedInterestRate
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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

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- Definition The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.16) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 20, 22 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.16) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
Name: | us-gaap_LineOfCredit
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Date the credit facility terminates, in CCYY-MM-DD format. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 22 -Article 5 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

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- Definition The effective interest rate at the end of the reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 22 -Article 5 + Details
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Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Details
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Balance Type: | na
Period Type: | duration

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- Definition Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 22 -Article 5 + 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 cash inflow from a contractual arrangement with the lender, including letter of credit, standby letter of credit and revolving credit arrangements, under which borrowings can be made up to a specific amount at any point in time with maturities due beyond one year or the operating cycle, if longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Financing Activities -URI http://asc.fasb.org/extlink&oid=6513228 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 14 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3255-108585 + Details
Name: | us-gaap_ProceedsFromLongTermLinesOfCredit
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Amount of cash outflow for payment of a long-term obligation from a lender, including but not limited to, letter of credit, standby letter of credit and revolving credit arrangements. + References No definition available. + Details
Name: | wern_Creditfacilityrepaymentslongterm
Namespace Prefix: | wern_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Line of Credit Facility, Number of Covenants + References No definition available. + Details
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Namespace Prefix: | wern_
Data Type: | xbrli:integerItemType
Balance Type: | na
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- Details
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- Details
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- Details
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