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Goodwill and Other Intangible Assets - Schedule of Goodwill by Segment (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Goodwill [Roll Forward] |  |  |
Beginning balance | $ 2,455.0 | $ 2,500.4 | $ 2,217.6
Foreign currency translation | 24.6 | (45.4) | (22.4)
Acquisition |  |  | 305.2
CDW Advanced Services Allocation |  | 0.0 |
Ending balance | 2,479.6 | 2,455.0 | 2,500.4
Corporate |  |  |
Goodwill [Roll Forward] |  |  |
Beginning balance | 1,074.1 | 1,045.9 | 1,045.9
Foreign currency translation | 0.0 | 0.0 | 0.0
Acquisition |  |  | 0.0
CDW Advanced Services Allocation |  | 28.2 |
Ending balance | 1,074.1 | 1,074.1 | 1,045.9
Accumulated impairment losses |  | (1,571.4) |
Small Business |  |  |
Goodwill [Roll Forward] |  |  |
Beginning balance | 185.9 | 185.9 | 185.9
Foreign currency translation | 0.0 | 0.0 | 0.0
Acquisition |  |  | 0.0
CDW Advanced Services Allocation |  | 0.0 |
Ending balance | 185.9 | 185.9 | 185.9
Public |  |  |
Goodwill [Roll Forward] |  |  |
Beginning balance | 929.6 | 911.3 | 911.3
Foreign currency translation | 0.0 | 0.0 | 0.0
Acquisition |  |  | 0.0
CDW Advanced Services Allocation |  | 18.3 |
Ending balance | 929.6 | 929.6 | 911.3
Accumulated impairment losses |  | (354.1) |
Other |  |  |
Goodwill [Roll Forward] |  |  |
Beginning balance | 265.4 | 357.3 | 74.5
Foreign currency translation | 24.6 | (45.4) | (22.4)
Acquisition |  |  | 305.2
CDW Advanced Services Allocation |  | (46.5) |
Ending balance | $ 290.0 | 265.4 | $ 357.3
Accumulated impairment losses |  | $ (28.3) |

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