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Equity Method Investments in Affiliates - Aggregate Purchase Price Allocation (Details) - USD ($) $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2015
Schedule of Equity Method Investments [Line Items] |  |  |  |
Deferred tax liability |  | $ (660.8) | $ (467.4) |
Goodwill |  | 2,628.1 | 2,662.5 | $ 2,668.4
Acquired Client Relationships (Net) |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Definite-lived acquired client relationships(1) |  | 501.9 | 421.0 | 621.4
Indefinite-lived acquired client relationships |  | 995.5 | $ 1,028.7 | $ 1,065.0
2016 New Seven Equity Method Investments | Equity Method Investee |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Tangible assets |  | 2.0 |  |
Deferred tax liability |  | (91.8) |  |
Goodwill |  | 854.4 |  |
Consideration paid |  | 1,362.3 |  |
2016 New Seven Equity Method Investments | Equity Method Investee | Acquired Client Relationships (Net) |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Definite-lived acquired client relationships(1) | [1] | 560.8 |  |
Indefinite-lived acquired client relationships |  | $ 36.9 |  |
Expected economic benefit period of acquired client relationship intangibles |  | 15 years |  |
[1] | The expected period of economic benefit utilized in the purchase price allocation for these definite-lived acquired client relationships was 15 years.

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