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Income Taxes - Narrative (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2017 USD ($)
Operating Loss Carryforwards [Line Items] |
Income tax benefit from change in tax rate | $ 96.0
Income tax expense from transition tax for accumulated foreign earnings | 20.0
Income tax expense from state impact | 1.0
Deferred income tax expense related to basis difference in CDW UK | 19.0
Deferred tax liability on unremitted CDW UK earnings | 3.0
State and Local Jurisdiction |
Operating Loss Carryforwards [Line Items] |
Operating loss carryforwards | 29.0
State tax credit carryforwards | 28.0
Tax Cuts And Jobs Act Of 2017 |
Operating Loss Carryforwards [Line Items] |
Valuation allowance | 14.0
State tax credit carryforwards | $ 14.0

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- Definition Amount of operating loss carryforward, before tax effects, available to reduce future taxable income under enacted tax laws. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32559-109319 + Details
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