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Equity Method Investments in Affiliates - Additional Information (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2019 | Jun. 30, 2019 | Mar. 31, 2019 | Sep. 30, 2018 | Sep. 30, 2019 | Sep. 30, 2018
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Amortization expense during the period | $ 21.1 |  |  | $ 30.1 | $ 72.0 | $ 76.5
Estimated Future Amortization Expense of Finite-Lived Intangibles |  |  |  |  |  |
Expense recognized to reduce the carrying amount of equity method investment | $ 10.0 |  | $ 415.0 |  |  |
Fair value assumptions, period of projected annual based fee margin growth rate |  |  | 5 years |  |  |
Fair value assumptions, projected annual based fee margin growth rate | (20.00%) |  | (13.00%) |  |  |
Fair value assumptions, discount rate for asset based fees | 11.00% |  | 11.00% |  |  |
Fair value assumptions, discount rate for performance based fees | 20.00% |  | 20.00% |  |  |
Fair value assumptions, market participant tax rate | 25.00% |  | 25.00% |  |  |
Acquired Client Relationships |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Amortization expense during the period |  |  |  |  | 72.0 |
Estimated Future Amortization Expense of Finite-Lived Intangibles |  |  |  |  |  |
Finite-lived intangible, future amortization expense in 2019 | $ 100.0 |  |  |  | 100.0 |
Finite-lived intangible, future amortization expense in 2020 | 75.0 |  |  |  | 75.0 |
Finite-lived intangible, future amortization expense in 2021 | 50.0 |  |  |  | 50.0 |
Finite-lived intangible, future amortization expense in 2022 | 50.0 |  |  |  | 50.0 |
Finite-lived intangible, future amortization expense in 2023 | 50.0 |  |  |  | 50.0 |
Equity Method Investee | Non-US |  |  |  |  |  |
Estimated Future Amortization Expense of Finite-Lived Intangibles |  |  |  |  |  |
Noncontrolling interest, sell of ownership percentage |  |  | 5.00% |  |  |
Payments to noncontrolling interest for additional equity interest in affiliate |  | $ 25.7 |  |  |  |
Equity Method Investee | Acquired Client Relationships |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Amortization expense during the period | 42.1 |  |  | $ 22.2 | 104.1 | $ 109.0
Estimated Future Amortization Expense of Finite-Lived Intangibles |  |  |  |  |  |
Finite-lived intangible, future amortization expense in 2019 | 150.0 |  |  |  | 150.0 |
Finite-lived intangible, future amortization expense in 2020 | 150.0 |  |  |  | 150.0 |
Finite-lived intangible, future amortization expense in 2021 | 75.0 |  |  |  | 75.0 |
Finite-lived intangible, future amortization expense in 2022 | 75.0 |  |  |  | 75.0 |
Finite-lived intangible, future amortization expense in 2023 | $ 75.0 |  |  |  | $ 75.0 |

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