v2.4.0.8

Shareholders Equity (Details) (USD $) | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended |  | 9 Months Ended | 3 Months Ended | 12 Months Ended | 3 Months Ended | 12 Months Ended | 1 Months Ended
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Jun. 30, 2013 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Sep. 30, 2013 | Sep. 30, 2014 | Sep. 30, 2013 | Dec. 31, 2013 | Jul. 02, 2013 | Sep. 30, 2014 Coworker Stock Purchase Plan [Member] | Sep. 30, 2014 Secondary offering over-allotment [Member] | Jun. 30, 2014 Secondary offering over-allotment [Member] | Mar. 31, 2014 Secondary offering over-allotment [Member] | Dec. 31, 2013 Secondary offering over-allotment [Member] | Sep. 30, 2014 Secondary Offering [Member] | Jun. 30, 2014 Secondary Offering [Member] | Mar. 31, 2014 Secondary Offering [Member] | Dec. 31, 2013 Secondary Offering [Member] | Dec. 31, 2013 IPO Over-allotment [Member] | Jul. 31, 2013 IPO [Member]
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Discount from Market Price, Purchase Date |  |  |  |  |  | 5.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Allocated Share-based Compensation Expense |  | $ 3,900,000 |  |  | $ 40,300,000 |  | $ 44,300,000 |  |  | $ 0.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stock Issued During Period, Shares, New Issues |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3,487,500 | 23,250,000
Share Price |  |  |  |  |  |  |  |  | $ 17.00 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from Issuance Initial Public Offering |  |  |  |  |  |  |  | 424,700,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares sold by certain selling stockholders |  |  |  |  |  |  |  |  |  |  | 0 | [1],[2] | 2,250,000 | [1] | 1,500,000 | [1] | 2,250,000 | [1] | 15,000,000 | 15,000,000 | 10,000,000 | 15,000,000 |  |
Secondary-offering related expenses |  | 300,000 | 500,000 | 400,000 |  | 1,200,000 | 0 | 600,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from shares sold by certain selling stockholders |  | $ 0 |  |  |  |  |  | $ 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders' Equity Note, Stock Split, Conversion Ratio | 143.0299613 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common shares, par value |  | $ 0.01 |  |  |  | $ 0.01 |  | $ 0.01 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
[1] | Under each underwriting agreement, the selling stockholders granted the underwriters an option, exercisable for thirty days, to purchase up to the additional amount of shares noted.
[2] | The option to purchase additional shares was not exercised in connection with the September 2014 secondary offering.

X
- Definition Employee Stock Purchase Plan, discount to market price, percent + References No definition available. + Details
Name: | cdw_EmployeeStockPurchasePlandiscounttomarketpricepercent
Namespace Prefix: | cdw_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Proceeds from shares sold by certain selling stockholders + References No definition available. + Details
Name: | cdw_Proceedsfromsharessoldbycertainsellingstockholders
Namespace Prefix: | cdw_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Secondary-offering related expenses + References No definition available. + Details
Name: | cdw_Secondaryofferingrelatedexpenses
Namespace Prefix: | cdw_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Shares sold by certain selling stockholders + References No definition available. + Details
Name: | cdw_Sharessoldbycertainsellingstockholders
Namespace Prefix: | cdw_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 14.F) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 14 -Section F + Details
Name: | us-gaap_AllocatedShareBasedCompensationExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | us-gaap_ClassOfStockLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Face amount or stated value per share of common stock. + 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 210.5-02.29) -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 30 -Article 5 + Details
Name: | us-gaap_CommonStockParOrStatedValuePerShare
Namespace Prefix: | us-gaap_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | instant

X
- Definition The cash inflow associated with the amount received from entity's first offering of stock to the public. + 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 (a) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3255-108585 + Details
Name: | us-gaap_ProceedsFromIssuanceInitialPublicOffering
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Price of a single share of a number of saleable stocks of a company. + References No definition available. + Details
Name: | us-gaap_SharePrice
Namespace Prefix: | us-gaap_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | instant

X
- Definition Ratio applied to the conversion of stock split, for example but not limited to, one share converted to two or two shares converted to one. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Stock Split -URI http://asc.fasb.org/extlink&oid=6525746 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 4 -Subparagraph (SAB TOPIC 4.C) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187143-122770 + Details
Name: | us-gaap_StockholdersEquityNoteStockSplitConversionRatio1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:pureItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of new stock issued during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Article 3 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 29, 30 -Article 5 + Details
Name: | us-gaap_StockIssuedDuringPeriodSharesNewIssues
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration