Credit Facilities (Narrative) (Details) (USD $) In Millions, unless otherwise specified			6 Months Ended
Jul. 31, 2012	Jun. 30, 2012	Jun. 30, 2012 Mature On May 31, 2016 Debt_Covenants	Jun. 30, 2012 Mature On May 31, 2017 Debt_Covenants
Line of Credit Facility [Line Items]
Total of committed credit facilities with two banks		$ 250	$ 175	$ 75
Committed credit facilities maturity			May 31, 2016	May 31, 2017
Term of credit facility (years)			4 years	5 years
Borrowings outstanding	20	0
Stand-by letters of credit		$ 35.9
Number of financial covenants for debt agreements			2	2

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

No definition available.

Name:	us-gaap_LettersOfCreditOutstandingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount borrowed under the credit facility as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

+ Details

Name:	us-gaap_LineOfCreditFacilityAmountOutstanding
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Reflects when the credit facility terminates, which may be presented in a variety of ways (year, month and year, day, month and year, quarter).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 129

-Paragraph 2, 4

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

Reference 3: 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

+ Details

Name:	us-gaap_LineOfCreditFacilityExpirationDate
Namespace Prefix:	us-gaap_
Data Type:	us-types:dateStringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_LineOfCreditFacilityLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- 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

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line of Credit Facility, Number of Covenants

+ References

No definition available.

+ Details

Name:	wern_LineOfCreditFacilityNumberOfCovenants
Namespace Prefix:	wern_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Line of Credit Facility, Term of Agreement

+ References

No definition available.

+ Details

Name:	wern_LineOfCreditFacilityTermOfAgreement
Namespace Prefix:	wern_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration