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Equity Method Investments in Affiliates - Additional Information (Details) - USD ($) | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Fair value assumptions, average projected growth rate |  | 10.00% |  |  |  |
Fair value assumptions, discount rate for asset based fees |  | 14.00% |  |  |  |
Fair value assumptions, discount rate for performance based fees |  | 25.00% |  |  |  |
Fair value assumptions, market participant tax rate |  | 25.00% |  |  |  |
Impairment of equity method investment |  | $ 93,100,000 |  |  |  |
Equity method and cost method impairments |  |  | $ 0 |  |  |
Revenue of equity method investment affiliate investment | [1] | 3,126,300,000 | 2,200,900,000 | [2] | $ 2,217,100,000 | [2]
Net income of equity method affiliate investment | [1] | 2,182,700,000 | 1,068,900,000 | [2] | 431,500,000 | [2]
Acquired Client Relationships (Net) |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Intangible amortization expense |  | 86,400,000 | 107,700,000 |  | 115,400,000 |
Intangible future amortization expense in 2018 |  | 85,000,000 |  |  |  |
Intangible future amortization expense in 2019 |  | 85,000,000 |  |  |  |
Intangible future amortization expense in 2020 |  | 50,000,000 |  |  |  |
Intangible future amortization expense in 2021 |  | 30,000,000 |  |  |  |
Intangible future amortization expense in 2022 |  | 30,000,000 |  |  |  |
Acquired Client Relationships Under Equity Method Investments |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Intangible amortization expense |  | 106,100,000 | 59,200,000 |  | $ 34,300,000 |
Intangible future amortization expense in 2018 |  | 120,000,000 |  |  |  |
Intangible future amortization expense in 2019 |  | 120,000,000 |  |  |  |
Intangible future amortization expense in 2020 |  | 120,000,000 |  |  |  |
Intangible future amortization expense in 2021 |  | 120,000,000 |  |  |  |
Intangible future amortization expense in 2022 |  | 120,000,000 |  |  |  |
Equity Method Investee |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Revenue of equity method investment affiliate investment |  | 1,317,800,000 | 944,100,000 |  |  |
Net income of equity method affiliate investment |  | 806,600,000 | 529,000,000 |  |  |
2016 New Seven Equity Method Investments | Equity Method Investee | Acquired Client Relationships (Net) |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Intangible amortization expense |  | $ 58,000,000 | $ 17,000,000 |  |  |
[1] | Revenue and the associated Net income include asset and performance based fees and the impact of consolidated investment products.
[2] | Revenue and Net income reflect investments in new Affiliates for the full-year, regardless of the date of the Company’s investment.

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