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Long-Term Debt - Narrative (Details) £ in Millions, $ in Millions			12 Months Ended
	Aug. 13, 2020 USD ($)	Sep. 26, 2019 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2020 GBP (£)	Apr. 21, 2020 USD ($)	Dec. 31, 2019 USD ($)
Debt Instrument [Line Items]
Long-term debt			$ 3,954.4			$ 3,337.9
CDW UK revolving credit facility
Debt Instrument [Line Items]
Long-term debt			$ 0.0			$ 0.0
Interest rate in effect			0.00%	0.00%		0.00%
Senior secured asset-based revolving credit facility
Debt Instrument [Line Items]
Undrawn letters of credit			$ 1.0
Borrowing base			2,200.0
Additional borrowing capacity			1,000.0
Long-term debt			0.0			$ 51.0
Senior secured asset-based revolving credit facility | CDW UK revolving credit facility
Debt Instrument [Line Items]
Borrowing capacity of the Revolving Loan			68.0	£ 50
CDW UK term loan
Debt Instrument [Line Items]
Amount of restricted payment capacity under the loan			218.0	£ 159
Long-term debt			$ 56.0			$ 61.0
Interest rate in effect			1.445%	1.445%		2.19%
Senior secured term loan facility
Debt Instrument [Line Items]
Amount of restricted payment capacity under the loan			$ 2,200.0
Periodic payment, principal			4.0
Long-term debt			$ 1,423.4			$ 1,438.3
Interest rate in effect			1.90%	1.90%		3.55%
Senior notes due 2028
Debt Instrument [Line Items]
Long-term debt			$ 600.0			$ 600.0
Interest rate in effect			4.25%	4.25%		4.25%
Senior Notes due 2023
Debt Instrument [Line Items]
Long-term debt		$ 525.0
Interest rate, stated percentage		5.00%
Percentage of principal amount redeemed		102.50%
Unsecured Debt | Senior Notes due 2029
Debt Instrument [Line Items]
Debt instrument, aggregate principal amount	$ 700.0
Interest rate, stated percentage	3.25%
Unsecured Debt | Senior notes due 2028
Debt Instrument [Line Items]
Debt instrument, aggregate principal amount		$ 600.0
Interest rate, stated percentage		4.25%
Unsecured Debt | Senior Notes due May 2025
Debt Instrument [Line Items]
Debt instrument, aggregate principal amount					$ 600.0
Interest rate, stated percentage					4.125%
Senior Notes due September 2025
Debt Instrument [Line Items]
Long-term debt	$ 600.0		$ 0.0			$ 600.0
Interest rate in effect			0.00%	0.00%		5.00%
Interest rate, stated percentage	5.00%
Percentage of principal amount redeemed	103.75%
Accounts Payable, Inventory Financing | Senior secured asset-based revolving credit facility
Debt Instrument [Line Items]
Amount owed under Revolving loan financing agreement			$ 459.0

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