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Goodwill and Acquired Client Relationships - Schedule of Changes in Acquired Client Relationships (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Net Book Value |  |  |
Net book value balance at the beginning of the period | $ 1,497.4 |  |
Intangible amortization and impairments | (86.4) | $ (110.2) | $ (115.4)
Net book value balance at the end of the period | 1,449.7 | 1,497.4 |
Acquired Client Relationships (Net) |  |  |
Definite-lived |  |  |
Gross book value, balance at the beginning of the period | 1,290.0 | 1,301.8 |
Accumulated amortization, balance at the beginning of the period | (788.1) | (680.4) |
Net book value, balance at the beginning of the period | 501.9 | 621.4 |
Intangible amortization and impairments | (86.4) | (107.7) |
Foreign currency translation | 5.5 | (11.8) |
Gross book value, balance at the end of the period | 1,295.5 | 1,290.0 | 1,301.8
Accumulated amortization, balance at the end of the period | (874.5) | (788.1) | (680.4)
Net book value, balance at the end of the period | 421.0 | 501.9 | 621.4
Indefinite-lived |  |  |
Net book value, balance at the beginning of the period | 995.5 | 1,065.0 |
Impairment | 0.0 | (2.5) |
Foreign currency translation | 33.2 | (67.0) |
Net book value, balance at the end of the period | 1,028.7 | 995.5 | 1,065.0
Net Book Value |  |  |
Net book value balance at the beginning of the period | 1,497.4 | 1,686.4 |
Intangible amortization and impairments | (86.4) | (110.2) |
Foreign currency translation | 38.7 | (78.8) |
Net book value balance at the end of the period | $ 1,449.7 | $ 1,497.4 | $ 1,686.4

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