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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 |  |  |

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