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Segment and Geographic Information - Revenues and Assets by Geographic Location (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Segment Reporting Information |  |  |  |  |  |  |  |  |  |  |  |
Revenue | $ 604.1 | [1] | $ 585.7 | $ 570.9 | $ 544.3 | $ 550.3 | $ 544.7 | $ 554.1 | $ 545.4 | $ 2,305.0 | $ 2,194.6 | $ 2,484.5
Fixed Assets, Net | 111.0 |  |  |  |  | 110.1 |  |  |  | 111.0 | 110.1 | 114.1
United States |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information |  |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  | 1,571.4 | 1,477.5 | 1,657.2
Fixed Assets, Net | 97.8 |  |  |  |  | 97.3 |  |  |  | 97.8 | 97.3 | 98.6
United Kingdom |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information |  |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  | 587.3 | 566.4 | 645.3
Fixed Assets, Net | 11.4 |  |  |  |  | 9.9 |  |  |  | 11.4 | 9.9 | 12.3
Other |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information |  |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  | 146.3 | 150.7 | 182.0
Fixed Assets, Net | $ 1.8 |  |  |  |  | $ 2.9 |  |  |  | $ 1.8 | $ 2.9 | $ 3.2
[1] | In the fourth quarter of 2017, the Company recorded a provisional one-time net benefit from changes in U.S tax laws (see Note 22) and an expense associated with the impairment of one of its Affiliates accounted for under the equity method (see Note 13).

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