Long-Term Debt - Term Loan (Details) £ in Millions, $ in Millions		3 Months Ended		12 Months Ended
	Aug. 01, 2016 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2018 GBP (£)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2017 GBP (£)	Feb. 28, 2017	Aug. 01, 2016 GBP (£)
Debt Instrument [Line Items]
Debt, long-term and short-term				$ 3,255.9	$ 3,267.8
Net loss on extinguishments of long-term debt				(57.4)	(2.1)	$ (24.3)
Senior secured term loan facility
Debt Instrument [Line Items]
Debt, long-term and short-term				1,468.0	$ 1,483.0
Debt issuance costs				$ 2.0
Quarterly amortization payment of original principal, Percent				0.25%			0.25%
Net loss on extinguishments of long-term debt				$ (14.0)
Write-off of unamortized deferred financing costs				5.0
Write-off of unamortized discount from prior term loan facility				9.0
Deferred financing fees incurred				$ 2.0
Interest rate in effect				3.70%	3.30%		3.70%
CDW UK term loan
Debt Instrument [Line Items]
Debt, long-term and short-term	$ 76.0			$ 75.7	$ 69.1				£ 56
Applicable margin				1.40%			1.40%
Debt instrument term	5 years
Capitalized fees as deferred financing costs				$ 1.0
Interest rate in effect				1.92%	1.80%		1.92%
CDW UK revolving credit facility
Debt Instrument [Line Items]
Debt, long-term and short-term				$ 0.0	$ 0.0		£ 0
Interest rate in effect				0.00%	0.00%		0.00%
Additional borrowing capacity				$ 68.0			£ 50
ABR | Senior secured term loan facility
Debt Instrument [Line Items]
Applicable margin				1.00%			1.00%
LIBOR | Senior secured term loan facility
Debt Instrument [Line Items]
Margin on borrowings, reduced interest rate from refinancing in bps								0.25%
Applicable margin				2.00%			2.00%
Scenario, Forecast | CDW UK term loan
Debt Instrument [Line Items]
Annual principal repayment installments		$ 7.0	£ 5