v2.4.1.9

Long-Term Debt Senior Notes due 2015 (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended			3 Months Ended		9 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2014	Mar. 31, 2014	Sep. 30, 2014	Dec. 01, 2014	Aug. 05, 2014	Mar. 20, 2014
Debt Instrument [Line Items]
Debt, long-term and short-term	$ 3,192.4us-gaap_DebtInstrumentCarryingAmount			$ 3,192.4us-gaap_DebtInstrumentCarryingAmount
Proceeds from issuance of long-term debt	1,175.0us-gaap_ProceedsFromIssuanceOfLongTermDebt	1,535.2us-gaap_ProceedsFromIssuanceOfLongTermDebt	135.7us-gaap_ProceedsFromIssuanceOfLongTermDebt
Gains (Losses) on Extinguishment of Debt	90.7us-gaap_GainsLossesOnExtinguishmentOfDebt	64.0us-gaap_GainsLossesOnExtinguishmentOfDebt	17.2us-gaap_GainsLossesOnExtinguishmentOfDebt
Write off of Deferred Debt Issuance Cost	12.6us-gaap_WriteOffOfDeferredDebtIssuanceCost	20.1us-gaap_WriteOffOfDeferredDebtIssuanceCost
Deferred financing costs, net	33.0us-gaap_DeferredFinanceCostsNoncurrentNet	30.1us-gaap_DeferredFinanceCostsNoncurrentNet	53.2us-gaap_DeferredFinanceCostsNoncurrentNet	33.0us-gaap_DeferredFinanceCostsNoncurrentNet
Senior notes due 2019 [Member]
Debt Instrument [Line Items]
Long-term debt, stated interest rate	8.50%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesDue2019Member			8.50%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesDue2019Member
Debt, long-term and short-term	503.9us-gaap_DebtInstrumentCarryingAmount / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesDue2019Member	1,305.0us-gaap_DebtInstrumentCarryingAmount / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesDue2019Member		503.9us-gaap_DebtInstrumentCarryingAmount / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesDue2019Member
Senior Notes, Amount Repurchased							541.4us-gaap_DebtInstrumentRepurchasedFaceAmount / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesDue2019Member	234.7us-gaap_DebtInstrumentRepurchasedFaceAmount / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesDue2019Member	25.0us-gaap_DebtInstrumentRepurchasedFaceAmount / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesDue2019Member
Extinguishment of Debt, Fees Paid to Lenders	23.0cdw_ExtinguishmentOfDebtFeesPaidToLenders / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesDue2019Member			10.0cdw_ExtinguishmentOfDebtFeesPaidToLenders / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesDue2019Member	2.4cdw_ExtinguishmentOfDebtFeesPaidToLenders / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesDue2019Member
Extinguishment of Debt, Accrued Interest Paid to Lenders									1.0cdw_ExtinguishmentOfDebtAccruedInterestPaidToLenders / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesDue2019Member
Gains (Losses) on Extinguishment of Debt	(36.9)us-gaap_GainsLossesOnExtinguishmentOfDebt / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesDue2019Member			(22.1)us-gaap_GainsLossesOnExtinguishmentOfDebt / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesDue2019Member		(2.7)us-gaap_GainsLossesOnExtinguishmentOfDebt / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesDue2019Member
Write off of Deferred Debt Issuance Cost	$ 4.7us-gaap_WriteOffOfDeferredDebtIssuanceCost / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesDue2019Member			$ 2.2us-gaap_WriteOffOfDeferredDebtIssuanceCost / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesDue2019Member	$ 0.3us-gaap_WriteOffOfDeferredDebtIssuanceCost / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesDue2019Member

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Extinguishment of Debt, Accrued Interest Paid to Lenders

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Amount of long-term debt before deduction of unamortized discount or premium. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt, with initial maturities beyond one year or the normal operating cycle, if longer.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Face (par) amount of the original debt instrument that was repurchased.

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Net amount of long-term deferred finance costs capitalized at the end of the reporting period.

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Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity.

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Write-off of amounts previously capitalized as debt issuance cost in an extinguishment of debt.

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