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Equity Method Investments in Affiliates - Additional Information (Details) - USD ($) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2018 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract] |  |  |  |  |  |
Fair value assumptions, average projected growth rate |  |  | 2.50% | 10.00% |  |
Fair value assumptions, discount rate for asset based fees |  |  | 11.00% | 14.00% |  |
Fair value assumptions, discount rate for performance based fees |  |  | 20.00% | 25.00% |  |
Fair value assumptions, market participant tax rate |  |  | 25.00% | 25.00% |  |
Impairment of equity method investment |  |  | $ 240,000,000 | $ 93,100,000 |  |
Equity method investments in Affiliates (net) |  |  | 2,791,000,000 | 3,304,700,000 |  |
Equity method and cost method impairments |  |  | 0 |  |  |
Undistributed earnings from equity method affiliates |  |  | 92,400,000 |  |  |
Revenue of equity method investment affiliate investment | [1] |  | 3,231,700,000 | 3,126,300,000 | $ 2,200,900,000 | [2]
Net income of equity method affiliate investment | [1] |  | 1,286,100,000 | 2,182,700,000 | 1,068,900,000 | [2]
Equity Method Investee |  |  |  |  |  |
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract] |  |  |  |  |  |
Equity method investments in Affiliates (net) |  |  | 2,791,000,000 | 3,304,700,000 | 3,368,300,000 |
Revenue of equity method investment affiliate investment |  |  | 1,137,500,000 | 1,317,800,000 |  |
Net income of equity method affiliate investment |  |  | 524,500,000 | 806,600,000 |  |
Equity Method Investee | Investment One |  |  |  |  |  |
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract] |  |  |  |  |  |
Impairment of equity method investment |  | $ 33,300,000 |  |  |  |
Equity method investments in Affiliates (net) |  | $ 0 | 0 |  |  |
Acquired Client Relationships Under Equity Method Investments |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Intangible amortization expense |  |  | 97,500,000 | $ 106,100,000 | $ 59,200,000 |
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract] |  |  |  |  |  |
Intangible future amortization expense in 2019 |  |  | 95,000,000 |  |  |
Intangible future amortization expense in 2020 |  |  | 95,000,000 |  |  |
Intangible future amortization expense in 2021 |  |  | 95,000,000 |  |  |
Intangible future amortization expense in 2022 |  |  | 75,000,000 |  |  |
Intangible future amortization expense in 2023 |  |  | $ 75,000,000 |  |  |
[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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- Definition The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6388964&loc=d3e16225-109274 + Details
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