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Long-Term Debt - Term Loan (Details)
£ in Millions, $ in Millions
3 Months Ended
Sep. 30, 2018
GBP (£)
Sep. 30, 2018
USD ($)
Mar. 31, 2018
USD ($)
Mar. 31, 2017
USD ($)
Mar. 31, 2018
GBP (£)
Mar. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Debt Instrument [Line Items]                
Carrying value           $ 3,255.0 $ 3,255.9  
Net loss on extinguishments of long-term debt     $ 0.0 $ 57.4        
Senior secured term loan facility                
Debt Instrument [Line Items]                
Carrying value           1,464.3 $ 1,468.0 $ 1,500.0
Deferred financing costs           $ 2.0    
Quarterly principal installment     $ 4.0          
Net loss on extinguishments of long-term debt       14.0        
Write off of deferred financing costs       5.0        
Write off of deferred debt discount       9.0        
Payments of debt issuance costs       $ 2.0        
Interest Rate         4.30% 4.30% 3.70%  
CDW UK term loan                
Debt Instrument [Line Items]                
Carrying value         £ 56.0 $ 78.5 $ 75.7  
Variable interest rate margin         1.40% 1.40%    
Deferred finance costs, net and unamortized premium         £ (1.0) $ (1.0)    
Interest Rate         1.98% 1.98% 1.92%  
CDW UK Revolving Credit Facility                
Debt Instrument [Line Items]                
Carrying value | £         £ 0.0      
Additional borrowing capacity         £ 50.0 $ 70.0    
ABR | Senior secured term loan facility                
Debt Instrument [Line Items]                
Variable interest rate margin         1.00% 1.00%    
LIBOR | Senior secured term loan facility                
Debt Instrument [Line Items]                
Variable interest rate margin         2.00% 2.00%    
Scenario, Forecast | CDW UK term loan                
Debt Instrument [Line Items]                
Repayments of debt £ 5.0 $ 7.0