Long-Term Debt -Debt Balances and Interest Rates (Details) £ in Millions, $ in Millions	Mar. 31, 2017 GBP (£)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Debt Instrument [Line Items]
Debt Instrument, Unused Borrowing Capacity, Fee in bps	25	25
Deferred Finance Costs, Net		$ (22.6)	$ (33.4)
Debt, short-term and long-term		3,303.1	3,267.8
Deferred financing costs, net		(22.6)	(33.4)
Current maturities of long-term debt		18.5	(18.5)
Deferred Finance Costs, Current, Net		0.0	0.0
Long-Term Debt, Current Maturities, Net of Deferred Financing Costs, Discount, or Premium		(18.5)	(18.5)
Long-term Debt, Excluding Current Maturities, Gross		3,284.6	3,249.3
Long-term Debt, Excluding Current Maturities		3,262.0	3,215.9
Long-Term Debt, Net of Deferred Financing Costs, Discount, or Premium		3,280.5	3,234.4
Revolving Loan [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Remaining Borrowing Capacity		963.0
Deferred Finance Costs, Net		0.0	0.0
Debt, short-term and long-term		$ 38.0	$ 0.0
Long-term Debt, Weighted Average Interest Rate	2.06%	2.06%	0.00%
Long-Term Debt, Net of Deferred Financing Costs, Discount, or Premium		$ 38.0	$ 0.0
CDW UK Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Remaining Borrowing Capacity	£ 50.0	63.0
Deferred Finance Costs, Net		0.0	0.0
Debt, short-term and long-term		$ 0.0	$ 0.0
Long-term Debt, Weighted Average Interest Rate	0.00%	0.00%	0.00%
Long-Term Debt, Net of Deferred Financing Costs, Discount, or Premium		$ 0.0	$ 0.0
Term Loan [Member]
Debt Instrument [Line Items]
Debt, short-term and long-term		1,479.2	1,483.0
Deferred Finance Costs, Net and Unamortized Premium		(5.0)
Deferred Finance Costs and Discount, Net		$ (2.3)	$ (14.9)
Long-term Debt, Weighted Average Interest Rate	3.15%	3.15%	3.30%
Long-Term Debt, Net of Deferred Financing Costs, Discount, or Premium		$ 1,476.9	$ 1,468.1
CDW UK Term Loan [Member]
Debt Instrument [Line Items]
Deferred Finance Costs, Net		(1.4)
Debt, short-term and long-term	£ 56.0	70.2	69.1
Deferred Finance Costs, Net and Unamortized Premium		(1.4)	(1.6)
Long-Term Debt, Net of Deferred Financing Costs, Discount, or Premium		68.8	67.5
Senior Notes due 2023 [Member]
Debt Instrument [Line Items]
Deferred Finance Costs, Net		(5.1)	(5.3)
Debt, short-term and long-term		$ 525.0	$ 525.0
Long-term Debt, Weighted Average Interest Rate	5.00%	5.00%	5.00%
Long-Term Debt, Net of Deferred Financing Costs, Discount, or Premium		$ 519.9	$ 519.7
Senior Notes due 2024 [Member]
Debt Instrument [Line Items]
Deferred Finance Costs, Net		(5.8)	(6.0)
Debt, short-term and long-term		$ 575.0	$ 575.0
Long-term Debt, Weighted Average Interest Rate	5.50%	5.50%	5.50%
Long-Term Debt, Net of Deferred Financing Costs, Discount, or Premium		$ 569.2	$ 569.0
Senior Notes due 2025 [Member]
Debt Instrument [Line Items]
Deferred Finance Costs, Net		(8.0)
Debt, short-term and long-term		$ 600.0
Long-term Debt, Weighted Average Interest Rate	5.00%	5.00%
Long-Term Debt, Net of Deferred Financing Costs, Discount, or Premium		$ 592.0
Other long-term obligations [Domain]
Debt Instrument [Line Items]
Deferred Finance Costs, Net		0.0	0.0
Debt, short-term and long-term		15.7	15.7
Long-Term Debt, Net of Deferred Financing Costs, Discount, or Premium		$ 15.7	$ 15.7
LIBOR [Member] | CDW UK Term Loan [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate	1.76%	1.76%	1.80%
Accounts Payable, Inventory Financing [Member]
Debt Instrument [Line Items]
Revolving Loan financing agreement		$ 427.2	$ 558.3