Long-Term Debt - Debt Covenants (Details) £ in Millions, $ in Millions	Mar. 31, 2018 GBP (£)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Debt Instrument [Line Items]
Long-term Debt, Gross		$ 3,255.0	$ 3,255.9
Senior secured term loan facility
Debt Instrument [Line Items]
Long-term Debt, Gross		1,464.3	1,468.0	$ 1,500.0
Amount of restricted payment capacity		$ 1,200.0
Net leverage ratio	2.49	2.49
CDW UK term loan
Debt Instrument [Line Items]
Long-term Debt, Gross	£ 56	$ 78.5	$ 75.7
Amount of restricted payment capacity	£ 99	$ 139.0