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Long-Term Debt - Debt Covenants (Details) £ in Millions, $ in Millions | Sep. 30, 2018 GBP (£) | Sep. 30, 2018 USD ($) | Apr. 03, 2018 | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
--- | --- | --- | --- | --- | ---
Debt Instrument [Line Items] |  |  |  |  |
Long-term Debt, Gross |  | $ 3,238.2 |  | $ 3,255.9 |
Senior secured term loan facility |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term Debt, Gross |  | 1,456.9 |  | 1,468.0 | $ 1,500.0
Amount of restricted payment capacity |  | $ 1,400.0 |  |  |
Net leverage ratio | 2.33 | 2.33 | 3.75 |  |
CDW UK term loan |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term Debt, Gross | £ 51 | $ 66.0 |  | $ 75.7 |
Amount of restricted payment capacity | £ 117 | $ 152.0 |  |  |

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- Definition Amount of restricted net assets of consolidated and unconsolidated subsidiaries as of the end of the most recently completed fiscal year. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(e)(3)(ii)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 + Details
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- Definition Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6031897-161870 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(16)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(22)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(16)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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