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Investments in Affiliates and Affiliate Sponsored Investment Products - Affiliated Sponsored Investment Products (Details) - Affiliate sponsored investment products - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
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Variable Interest Entity [Line Items] |  |
Unconsolidated VIE Net Assets | $ 2,154.6 | $ 1,756.6
Carrying Value and Maximum Risk of Loss | $ 10.2 | $ 9.4

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