Investment Securities - Sponsored Privately Offered Funds (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investment Securities
Proceeds from maturities of trading debt securities	$ 84.5	$ 237.2	$ 234.4
Net realized gains on sale of equity securities	0.3
Net realized gains (losses) on sale of trading securities		(0.5)
Proceeds from sale of equity securities	8.3
Proceeds from sale of trading securities		19.8
Net realized gains on sale of available for sale securities		0.9	3.6
Proceeds from sale of available for sale securities		86.9	98.2
Consolidated Sponsored Funds
Investment Securities
Net realized gains (losses) on sale of trading securities	12.8	1.5
Proceeds from sale of trading securities	75.1	57.1
Sponsored funds
Investment Securities
Net realized gain (loss) on equity method investments		6.9	(2.3)
Proceeds from sale of equity method investments	1.2	73.2	$ 58.7
Sponsored funds \| Maximum
Investment Securities
Net realized gain (loss) on equity method investments	0.1
Sponsored funds \| Investment and other income (loss)
Investment Securities
Pre-tax charge to reflect the “other than temporary” decline in value of investments	$ 0.3	$ 1.3

Amount of realized gain on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (a)
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Amount of realized gain (loss) on investment in debt security measured at fair value with change in fair value recognized in net income (trading).

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Amount of realized gain from sale of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Amount of cash inflow from sale of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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The amount of cash inflow from sale of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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