Long-Term Debt - Debt Balances and Interest Rates (Details) £ in Millions	Sep. 30, 2018 GBP (£)	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Debt Instrument [Line Items]
Principal		$ 3,238,200,000	$ 3,255,900,000
Long-term debt, current maturities, gross		(32,000,000.0)	(25,500,000)
Long-term debt, excluding current maturities, gross		3,206,200,000	3,230,400,000
Deferred finance costs, net		(18,800,000)	(20,400,000)
Deferred finance costs, current, net		0	0
Deferred financing costs, noncurrent, net		(18,800,000)	(20,400,000)
Long-term debt, net of deferred financing costs		3,219,400,000	3,235,500,000
Current maturities of long-term debt, net of Deferred financing costs		(32,000,000.0)	(25,500,000)
Long-term debt, excluding current maturities, net of deferred financing costs		$ 3,187,400,000	$ 3,210,000,000.0
Senior secured asset-based revolving credit facility
Debt Instrument [Line Items]
Interest Rate	0.00%	0.00%	0.00%
Principal		$ 0	$ 0
Deferred finance costs, net		0	0
Long-term debt, net of deferred financing costs		$ 0	$ 0
CDW UK revolving credit facility
Debt Instrument [Line Items]
Interest Rate	2.10%	2.10%	0.00%
Principal		$ 6,500,000	$ 0
Deferred finance costs, net		0	0
Long-term debt, net of deferred financing costs		$ 6,500,000	$ 0
Senior secured term loan facility
Debt Instrument [Line Items]
Interest Rate	3.80%	3.80%	3.70%
Principal		$ 1,456,900,000	$ 1,468,000,000.0	$ 1,500,000,000
Deferred finance costs, net		(2,700,000)	(2,000,000.0)
Long-term debt, net of deferred financing costs		$ 1,454,200,000	$ 1,466,000,000.0
CDW UK term loan
Debt Instrument [Line Items]
Interest Rate	2.29%	2.29%	1.90%
Principal	£ 51	$ 66,000,000	$ 75,700,000
Deferred finance costs, net		(1,100,000)	(1,400,000)
Long-term debt, net of deferred financing costs		$ 65,400,000	$ 74,300,000
Senior notes due 2023
Debt Instrument [Line Items]
Interest Rate	5.00%	5.00%	5.00%
Principal		$ 525,000,000.0	$ 525,000,000.0
Deferred finance costs, net		(3,800,000)	(4,500,000)
Long-term debt, net of deferred financing costs		$ 521,200,000	$ 520,500,000
Senior notes due 2024
Debt Instrument [Line Items]
Interest Rate	5.50%	5.50%	5.50%
Principal		$ 575,000,000.0	$ 575,000,000.0
Deferred finance costs, net		(4,600,000)	(5,200,000)
Long-term debt, net of deferred financing costs		$ 570,400,000	$ 569,800,000
Senior notes due 2025
Debt Instrument [Line Items]
Interest Rate	5.00%	5.00%	5.00%
Principal		$ 600,000,000.0	$ 600,000,000.0
Deferred finance costs, net		(6,600,000)	(7,300,000)
Long-term debt, net of deferred financing costs		593,400,000	592,700,000
Other long-term obligations
Debt Instrument [Line Items]
Principal		8,300,000	12,200,000
Deferred finance costs, net		0	0
Long-term debt, net of deferred financing costs		$ 8,300,000	$ 12,200,000