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)