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Equity Investments (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Equity Investments [Abstract]      
Equity Method Investment, Ownership Percentage 35.00%us-gaap_EquityMethodInvestmentOwnershipPercentage    
Equity Method Investment, Percentage Available to be Acquired 65.00%cdw_EquityMethodInvestmentPercentageAvailabletobeAcquired    
Payments to Acquire Equity Method Investments $ 86.8us-gaap_PaymentsToAcquireEquityMethodInvestments $ 0us-gaap_PaymentsToAcquireEquityMethodInvestments $ 0us-gaap_PaymentsToAcquireEquityMethodInvestments
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity $ 119.2us-gaap_EquityMethodInvestmentDifferenceBetweenCarryingAmountAndUnderlyingEquity