Income Taxes - Narrative (Details) $ in Millions	12 Months Ended
	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