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Long-Term Debt - Term Loan (Details) £ in Millions, $ in Millions |  | 3 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Aug. 01, 2016 USD ($) | Sep. 30, 2018 USD ($) | Sep. 30, 2018 GBP (£) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2017 GBP (£) | Feb. 28, 2017 | Aug. 01, 2016 GBP (£)
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Debt, long-term and short-term |  |  |  | $ 3,255.9 | $ 3,267.8 |  |  |  |
Net loss on extinguishments of long-term debt |  |  |  | (57.4) | (2.1) | $ (24.3) |  |  |
Senior secured term loan facility |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Debt, long-term and short-term |  |  |  | 1,468.0 | $ 1,483.0 |  |  |  |
Debt issuance costs |  |  |  | $ 2.0 |  |  |  |  |
Quarterly amortization payment of original principal, Percent |  |  |  | 0.25% |  |  | 0.25% |  |
Net loss on extinguishments of long-term debt |  |  |  | $ (14.0) |  |  |  |  |
Write-off of unamortized deferred financing costs |  |  |  | 5.0 |  |  |  |  |
Write-off of unamortized discount from prior term loan facility |  |  |  | 9.0 |  |  |  |  |
Deferred financing fees incurred |  |  |  | $ 2.0 |  |  |  |  |
Interest rate in effect |  |  |  | 3.70% | 3.30% |  | 3.70% |  |
CDW UK term loan |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Debt, long-term and short-term | $ 76.0 |  |  | $ 75.7 | $ 69.1 |  |  |  | £ 56
Applicable margin |  |  |  | 1.40% |  |  | 1.40% |  |
Debt instrument term | 5 years |  |  |  |  |  |  |  |
Capitalized fees as deferred financing costs |  |  |  | $ 1.0 |  |  |  |  |
Interest rate in effect |  |  |  | 1.92% | 1.80% |  | 1.92% |  |
CDW UK revolving credit facility |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Debt, long-term and short-term |  |  |  | $ 0.0 | $ 0.0 |  | £ 0 |  |
Interest rate in effect |  |  |  | 0.00% | 0.00% |  | 0.00% |  |
Additional borrowing capacity |  |  |  | $ 68.0 |  |  | £ 50 |  |
ABR | Senior secured term loan facility |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Applicable margin |  |  |  | 1.00% |  |  | 1.00% |  |
LIBOR | Senior secured term loan facility |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Margin on borrowings, reduced interest rate from refinancing in bps |  |  |  |  |  |  |  | 0.25% |
Applicable margin |  |  |  | 2.00% |  |  | 2.00% |  |
Scenario, Forecast | CDW UK term loan |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Annual principal repayment installments |  | $ 7.0 | £ 5 |  |  |  |  |  |

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