Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Long-Term Debt - Debt Covenants (Details) £ in Millions, $ in Millions	Sep. 30, 2018 GBP (£)	Sep. 30, 2018 USD ($)	Apr. 03, 2018	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Debt Instrument [Line Items]
Long-term Debt, Gross		$ 3,238.2		$ 3,255.9
Senior secured term loan facility
Debt Instrument [Line Items]
Long-term Debt, Gross		1,456.9		1,468.0	$ 1,500.0
Amount of restricted payment capacity		$ 1,400.0
Net leverage ratio	2.33	2.33	3.75
CDW UK term loan
Debt Instrument [Line Items]
Long-term Debt, Gross	£ 51	$ 66.0		$ 75.7
Amount of restricted payment capacity	£ 117	$ 152.0