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Equity-Based Compensation Equity Awards Granted by Seller of CDW UK (Details) - USD ($) $ / shares in Units, $ in Millions | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Granted (in shares) | 1,021,398 |  |
Grants, weighted average exercise price (in dollars per share) | $ 73.85 |  |
Payment of incentive compensation plan withholding taxes | $ 33.9 | $ 49.6 | $ 0.0
CDW UK | Employee Stock Option |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Granted (in shares) | 456,613 |  |
Grants, weighted average exercise price (in dollars per share) | $ 35.93 |  |
Payment of incentive compensation plan withholding taxes | $ 19.0 |  |

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- Definition Gross number of share options (or share units) granted during the period. + References No definition available. + Details
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