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Investments in Marketable Securities - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Proceeds received from sales of marketable securities	$ 15.1	$ 13.8
Net realized gains (losses) on sale of marketable securities	0.5	$ 2.8
Affiliate sponsored investment products
Investment [Line Items]
Fair value of deconsolidated investment in affiliate	$ 84.3

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- Definition

Marketable Securities, Fair Value, Deconsolidation

+ References

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- Definition

Line items represent reporting concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 12
-Sentence Column A

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- Definition

Amount of realized gain (loss) on investment in marketable security, including other-than-temporary impairment (OTTI).

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The cash inflow associated with the aggregate amount received by the entity through sale or maturity of marketable securities (held-to-maturity or available-for-sale) during the period.

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