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Income Taxes - Narrative (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Operating Loss Carryforwards [Line Items]        
Deferred income tax expense related to basis difference in CDW UK $ 19.0      
Deferred tax liability on unremitted CDW UK earnings 1.0      
Unrecognized tax benefits 18.3 $ 17.7 $ 15.1 $ 0.0
State and Local Jurisdiction        
Operating Loss Carryforwards [Line Items]        
Operating loss carryforwards 6.0      
State tax credit carryforwards 23.0      
Unrecognized tax benefits, impact on effective tax rate, net of tax rate $ 15.0