Long-Term Debt - Debt Balances and Interest Rates (Details) £ in Millions, $ in Millions	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)	Aug. 01, 2016 GBP (£)
Debt Instrument [Line Items]
Long-term debt	$ 3,318.2	$ 3,267.8
Long-term debt, current maturities, gross	(18.5)	(18.5)
Long-term debt, excluding current maturities, gross	3,299.7	3,249.3
Deferred finance costs, net	21.9	33.4
Deferred finance costs, current, net	0.0	0.0
Deferred financing costs, noncurrent, net	(21.9)	(33.4)
Long-term debt, net of deferred financing costs	3,296.3	3,234.4
Current maturities of long-term debt, net of Deferred financing costs	(18.5)	(18.5)
Long-term debt, excluding current maturities, net of deferred financing costs	$ 3,277.8	$ 3,215.9
Senior secured asset-based revolving credit facility
Debt Instrument [Line Items]
Interest Rate	3.30%	0.00%
Long-term debt	$ 54.0	$ 0.0
Deferred finance costs, net	0.0	0.0
Long-term debt, net of deferred financing costs	$ 54.0	$ 0.0
CDW UK revolving credit facility
Debt Instrument [Line Items]
Interest Rate	0.00%	0.00%
Long-term debt	$ 0.0	$ 0.0
Deferred finance costs, net	0.0	0.0
Long-term debt, net of deferred financing costs	$ 0.0	$ 0.0
Senior secured term loan facility
Debt Instrument [Line Items]
Interest Rate	3.20%	3.30%
Long-term debt	$ 1,475.6	$ 1,483.0
Deferred finance costs, net	2.3	14.9
Long-term debt, net of deferred financing costs	$ 1,473.3	$ 1,468.1
CDW UK term loan
Debt Instrument [Line Items]
Interest Rate	1.69%	1.80%
Long-term debt	$ 72.9	$ 69.1	£ 56
Deferred finance costs, net	1.3	1.6
Long-term debt, net of deferred financing costs	$ 71.6	$ 67.5
Senior notes due 2022
Debt Instrument [Line Items]
Interest Rate		6.00%
Long-term debt		$ 600.0
Deferred finance costs, net		5.6
Long-term debt, net of deferred financing costs		$ 594.4
Senior notes due 2023
Debt Instrument [Line Items]
Interest Rate	5.00%	5.00%
Long-term debt	$ 525.0	$ 525.0
Deferred finance costs, net	4.9	5.3
Long-term debt, net of deferred financing costs	$ 520.1	$ 519.7
Senior notes due 2024
Debt Instrument [Line Items]
Interest Rate	5.50%	5.50%
Long-term debt	$ 575.0	$ 575.0
Deferred finance costs, net	5.6	6.0
Long-term debt, net of deferred financing costs	$ 569.4	569.0
Senior notes due 2025
Debt Instrument [Line Items]
Interest Rate	5.00%
Long-term debt	$ 600.0
Deferred finance costs, net	7.8
Long-term debt, net of deferred financing costs	592.2
Other long-term obligations
Debt Instrument [Line Items]
Long-term debt	15.7	15.7
Deferred finance costs, net	0.0	0.0
Long-term debt, net of deferred financing costs	$ 15.7	$ 15.7