Investments (Realized Gains Losses Available-For-Sale Securities) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Proceeds from Sales	$ 2.8	$ 43.8
Gross Realized Gains	0.0	1.9
Gross Realized Losses	(0.1)	(1.3)
Seed Money [Member]
Schedule of Available-for-sale Securities [Line Items]
Proceeds from Sales		34.2
Gross Realized Gains		1.6
Gross Realized Losses		(1.3)
Collateralized Loan Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Proceeds from Sales	2.6	2.0
Gross Realized Gains	0.0	0.3
Gross Realized Losses	0.0	0.0
Other Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Proceeds from Sales	0.2	7.6
Gross Realized Gains	0.0	0.0
Gross Realized Losses	$ (0.1)	$ 0.0

This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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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+ References

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- Definition

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26853-111562

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-Subparagraph (a),(b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3179-108585

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No definition available.

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