Investments (Realized Gains Losses Available-For-Sale Securities) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Debt Securities, Available-for-sale [Line Items]
Proceeds from Sales	$ 4.2	$ 18.8	$ 22.8	$ 76.4
Gross Realized Gains	1.5	2.6	2.3	4.9
Gross Realized Losses	0.0	0.0	(3.6)	(1.5)
Seed money
Debt Securities, Available-for-sale [Line Items]
Proceeds from Sales		15.2		61.7
Gross Realized Gains		1.6		2.7
Gross Realized Losses		0.0		(1.5)
CLOs
Debt Securities, Available-for-sale [Line Items]
Proceeds from Sales	4.2	3.5	16.5	6.1
Gross Realized Gains	1.5	0.8	2.3	1.2
Gross Realized Losses	0.0	0.0	0.0	0.0
Other debt securities
Debt Securities, Available-for-sale [Line Items]
Proceeds from Sales	0.0	0.1	6.3	8.6
Gross Realized Gains	0.0	0.2	0.0	1.0
Gross Realized Losses	$ 0.0	$ 0.0	$ (3.6)	$ 0.0

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)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563

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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).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26853-111562

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3151-108585

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-Topic 230
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-Paragraph 12
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3179-108585

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