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Equity-Based Compensation (Details) - USD ($) $ / shares in Units, $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Equity-based compensation [Abstract] |  |  |
Equity-based compensation expense | $ 42.5 | $ 48.5 | $ 40.7
Income tax benefit | (7.7) | (9.8) | (9.9)
Equity-based compensation expense, net of tax | 34.8 | $ 38.7 | 30.8
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized [Abstract] |  |  |
Compensation cost not yet recognized | $ 43.0 |  |
Compensation cost not yet recognized, period for recognition | 2 years |  |
Number of Options (in shares): |  |  |
Options outstanding, beginning (in shares) | 4,138,242 |  |
Granted (in shares) | 991,431 |  |
Forfeitures/Expired (in shares) | (44,409) |  |
Exercised (in shares) | (1,119,812) |  |
Options outstanding, ending (in shares) | 3,965,452 | 4,138,242 |
Options Weighted Average Exercise Price (in dollars per share): |  |  |
Options outstanding, beginning weighted-average exercise price (in dollars per share) | $ 59.39 |  |
Grants, weighted average exercise price (in dollars per share) | 100.80 |  |
Forfeitures and Expirations, weighted average exercise price (in dollars per share) | 92.54 |  |
Exercises, weighted average exercise price (in dollars per share) | 44.05 |  |
Options outstanding, ending weighted-average exercise price (in dollars per share) | $ 73.71 | $ 59.39 |
Options, Additional Disclosures: |  |  |
Options, exercisable (in shares) | 2,192,951 |  |
Options, vested and expected to vest, Outstanding (in shares) | 1,745,547 |  |
Options, exercisable, weighted average exercise price (in dollars per share) | $ 56.63 |  |
Options, vested and expected to vest, weighted average exercise price (in dollars per share) | $ 94.70 |  |
Options, outstanding, weighted average remaining contractual term | 6 years 5 months 23 days |  |
Options, exercisable, weighted average remaining contractual term (years) | 4 years 11 months 23 days |  |
Options, vested and expected to vest, outstanding, weighted average remaining contractual term (in years) | 8 years 3 months 25 days |  |
Options, outstanding intrinsic value | $ 230.3 |  |
Options, exercisable, Intrinsic Value | 164.8 |  |
Options, vested and expected to vest, aggregate intrinsic value | 64.7 |  |
Total intrinsic value of stock options exercised | $ 94.0 | $ 83.0 | $ 47.0
Employee Stock Option |  |  |
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized [Abstract] |  |  |
Award expiration period | 10 years |  |
Award vesting period | 3 years |  |
Fair Value Assumptions: |  |  |
Grant date fair value (in dollars per share) | $ 20.46 | $ 19.26 | $ 14.80
Volatility (as percent) | 25.50% | 20.00% | 20.00%
Risk-free rate (as percent) | 0.51% | 2.53% | 2.75%
Expected dividend yield (as percent) | 1.52% | 1.23% | 1.14%
Expected term (in years) | 6 years | 6 years | 6 years
Restricted Stock Units (RSUs) |  |  |
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized [Abstract] |  |  |
Award vesting period | 3 years |  |
Performance Share Units (PSUs) |  |  |
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized [Abstract] |  |  |
Award vesting period | 3 years |  |
Minimum | Performance Share Units (PSUs) |  |  |
Options, Additional Disclosures: |  |  |
Potential vesting percentage range of shares | 0.00% |  |
Maximum | Performance Share Units (PSUs) |  |  |
Options, Additional Disclosures: |  |  |
Potential vesting percentage range of shares | 200.00% |  |
2013 Long Term Incentive Plan |  |  |
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized [Abstract] |  |  |
Number of shares authorized (in shares) | 15,500,000 |  |
Options nonvested number of shares (in shares) | 3,800,000 |  |
Number of share available for grant (in shares) | 2,600,000 |  |
Equity Awards Granted by Seller of CDW UK |  |  |
Number of Options (in shares): |  |  |
Exercised (in shares) | (110,978) | (456,613) |

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