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Intangible Assets by Asset Type (Details) (USD $) In Millions, unless otherwise specified
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12 Months Ended |
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Dec. 31, 2014
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Dec. 31, 2013
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| Intangible Assets [Line Items] |
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| Gross Carrying Amount |
$ 2,394.0us-gaap_FiniteLivedIntangibleAssetsGross |
$ 2,413.4us-gaap_FiniteLivedIntangibleAssetsGross |
| Accumulated Amortization |
1,225.2us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization |
1,085.4us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization |
| Net Carrying Amount |
1,168.8us-gaap_FiniteLivedIntangibleAssetsNet |
1,328.0us-gaap_FiniteLivedIntangibleAssetsNet |
| Customer Relationships [Member] |
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| Intangible Assets [Line Items] |
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| Gross Carrying Amount |
1,859.7us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember |
1,860.8us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember |
| Accumulated Amortization |
1,012.1us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember |
872.8us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember |
| Net Carrying Amount |
847.6us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember |
988.0us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_CustomerRelationshipsMember |
| Trade Name [Member] |
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| Intangible Assets [Line Items] |
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| Gross Carrying Amount |
421.0us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_TradeNamesMember |
421.0us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_TradeNamesMember |
| Accumulated Amortization |
152.0us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_TradeNamesMember |
130.9us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_TradeNamesMember |
| Net Carrying Amount |
269.0us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_TradeNamesMember |
290.1us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_TradeNamesMember |
| Internally Developed Software [Member] |
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| Intangible Assets [Line Items] |
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| Gross Carrying Amount |
110.1us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_SoftwareAndSoftwareDevelopmentCostsMember |
128.5us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_SoftwareAndSoftwareDevelopmentCostsMember |
| Accumulated Amortization |
58.9us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_SoftwareAndSoftwareDevelopmentCostsMember |
79.8us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_SoftwareAndSoftwareDevelopmentCostsMember |
| Net Carrying Amount |
51.2us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_SoftwareAndSoftwareDevelopmentCostsMember |
48.7us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_SoftwareAndSoftwareDevelopmentCostsMember |
| Disposal of Fully Amortized Definite-Lived Intangible Assets |
41.7cdw_DisposalofFullyAmortizedDefiniteLivedIntangibleAssets / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_SoftwareAndSoftwareDevelopmentCostsMember |
0cdw_DisposalofFullyAmortizedDefiniteLivedIntangibleAssets / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_SoftwareAndSoftwareDevelopmentCostsMember |
| Other Intangible Assets [Member] |
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| Intangible Assets [Line Items] |
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| Gross Carrying Amount |
3.2us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_OtherIntangibleAssetsMember |
3.1us-gaap_FiniteLivedIntangibleAssetsGross / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_OtherIntangibleAssetsMember |
| Accumulated Amortization |
2.2us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_OtherIntangibleAssetsMember |
1.9us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_OtherIntangibleAssetsMember |
| Net Carrying Amount |
$ 1.0us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_OtherIntangibleAssetsMember |
$ 1.2us-gaap_FiniteLivedIntangibleAssetsNet / us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis = us-gaap_OtherIntangibleAssetsMember |