Investments (Gross Unrealized Holding Gains And Losses Recognized In Other Accumulated Comprehensive Income From Available-For-Sale Investments) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 9.9	$ 79.9
Gross Unrealized Holding Gains	1.4	6.6
Gross Unrealized Holding Losses	(0.2)	(1.3)
Available for sale investments	11.1	85.2
Seed Money [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost		65.1
Gross Unrealized Holding Gains		5.5
Gross Unrealized Holding Losses		(1.3)
Available for sale investments		69.3
CLO [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	3.5	4.9
Gross Unrealized Holding Gains	1.4	1.1
Gross Unrealized Holding Losses	0.0	0.0
Available for sale investments	4.9	6.0
Other Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	6.4	9.9
Gross Unrealized Holding Gains	0.0	0.0
Gross Unrealized Holding Losses	(0.2)	0.0
Available for sale investments	$ 6.2	$ 9.9

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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- Definition

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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No definition available.

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