Income Taxes - Narrative (Details) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)
Operating Loss Carryforwards [Line Items]
Provisional amount, tax benefit	$ 2
Deferred income tax expense related to basis difference in CDW UK		$ 19
Deferred tax liability on unremitted CDW UK earnings		3
Unrecognized tax benefits that would impact effective tax rate	15	15
Unrecognized tax benefits that would impact effective tax rate, net of tax rate	12	12
State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	11	11
State tax credit carryforwards	$ 25	$ 25