v2.4.1.9

Long-Term Debt Senior Notes (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]
Payments for Derivative Instrument, Investing Activities	$ 2.1us-gaap_PaymentsForDerivativeInstrumentInvestingActivities	$ 0us-gaap_PaymentsForDerivativeInstrumentInvestingActivities	$ 0.3us-gaap_PaymentsForDerivativeInstrumentInvestingActivities
Proceeds from issuance of long-term debt	1,175.0us-gaap_ProceedsFromIssuanceOfLongTermDebt	1,535.2us-gaap_ProceedsFromIssuanceOfLongTermDebt	135.7us-gaap_ProceedsFromIssuanceOfLongTermDebt
Debt, long-term and short-term	3,192.4us-gaap_DebtInstrumentCarryingAmount			3,192.4us-gaap_DebtInstrumentCarryingAmount
Deferred financing costs, net	33.0us-gaap_DeferredFinanceCostsNoncurrentNet	30.1us-gaap_DeferredFinanceCostsNoncurrentNet	53.2us-gaap_DeferredFinanceCostsNoncurrentNet	33.0us-gaap_DeferredFinanceCostsNoncurrentNet
Gains (Losses) on Extinguishment of Debt	(90.7)us-gaap_GainsLossesOnExtinguishmentOfDebt	(64.0)us-gaap_GainsLossesOnExtinguishmentOfDebt	(17.2)us-gaap_GainsLossesOnExtinguishmentOfDebt
Write off of Deferred Debt Issuance Cost	12.6us-gaap_WriteOffOfDeferredDebtIssuanceCost	20.1us-gaap_WriteOffOfDeferredDebtIssuanceCost
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
Redemption Premium, percentage of par value							106.202%cdw_RedemptionPremiumpercentageofparvalue / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesDue2019Member	108.764%cdw_RedemptionPremiumpercentageofparvalue / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesDue2019Member	109.75%cdw_RedemptionPremiumpercentageofparvalue / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesDue2019Member
Extinguishment of Debt, Accrued Interest Paid to Lenders									1.0cdw_ExtinguishmentOfDebtAccruedInterestPaidToLenders / 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
Premium, Unamortized	1.3us-gaap_DebtInstrumentUnamortizedPremium / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesDue2019Member	4.2us-gaap_DebtInstrumentUnamortizedPremium / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesDue2019Member		1.3us-gaap_DebtInstrumentUnamortizedPremium / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesDue2019Member
Debt Instrument, Repurchased Face Amount							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
Gains (Losses) on Extinguishment of Debt	36.9us-gaap_GainsLossesOnExtinguishmentOfDebt / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesDue2019Member			22.1us-gaap_GainsLossesOnExtinguishmentOfDebt / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesDue2019Member		2.7us-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
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, Make-whole Interest Payment	10.6cdw_ExtinguishmentofDebtMakewholeInterestPayment / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesDue2019Member			10.6cdw_ExtinguishmentofDebtMakewholeInterestPayment / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesDue2019Member
Write off of Deferred Debt issuance Cost, unamortized premium	1.4cdw_WriteoffofDeferredDebtissuanceCostunamortizedpremium / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesDue2019Member			0.7cdw_WriteoffofDeferredDebtissuanceCostunamortizedpremium / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesDue2019Member
Senior Notes due 2022 [Member]
Debt Instrument [Line Items]
Long-term debt, stated interest rate	6.00%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesdue2022Member			6.00%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesdue2022Member
Debt, long-term and short-term	600.0us-gaap_DebtInstrumentCarryingAmount / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesdue2022Member			600.0us-gaap_DebtInstrumentCarryingAmount / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesdue2022Member
Premium, Unamortized		0us-gaap_DebtInstrumentUnamortizedPremium / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesdue2022Member
Deferred financing costs, net	8.0us-gaap_DeferredFinanceCostsNoncurrentNet / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesdue2022Member			8.0us-gaap_DeferredFinanceCostsNoncurrentNet / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesdue2022Member
Senior Notes due 2024 [Member]
Debt Instrument [Line Items]
Long-term debt, stated interest rate	5.50%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesdue2024Member			5.50%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesdue2024Member
Debt, long-term and short-term	575.0us-gaap_DebtInstrumentCarryingAmount / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesdue2024Member			575.0us-gaap_DebtInstrumentCarryingAmount / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesdue2024Member
Premium, Unamortized		0us-gaap_DebtInstrumentUnamortizedPremium / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesdue2024Member
Deferred financing costs, net	7.5us-gaap_DeferredFinanceCostsNoncurrentNet / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesdue2024Member			7.5us-gaap_DeferredFinanceCostsNoncurrentNet / us-gaap_LongtermDebtTypeAxis = cdw_SeniorNotesdue2024Member
Cap agreement effective January 14, 2015-2017 [Member]
Debt Instrument [Line Items]
Payments for Derivative Instrument, Investing Activities	2.1us-gaap_PaymentsForDerivativeInstrumentInvestingActivities / us-gaap_DerivativeByNatureAxis = cdw_CapagreementeffectiveJanuary1420152017Member
Derivative Asset, Noncurrent	1.7us-gaap_DerivativeAssetsNoncurrent / us-gaap_DerivativeByNatureAxis = cdw_CapagreementeffectiveJanuary1420152017Member			1.7us-gaap_DerivativeAssetsNoncurrent / us-gaap_DerivativeByNatureAxis = cdw_CapagreementeffectiveJanuary1420152017Member
Derivative, Notional Amount	$ 1,000.0invest_DerivativeNotionalAmount / us-gaap_DerivativeByNatureAxis = cdw_CapagreementeffectiveJanuary1420152017Member			$ 1,000.0invest_DerivativeNotionalAmount / us-gaap_DerivativeByNatureAxis = cdw_CapagreementeffectiveJanuary1420152017Member

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

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