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Cash collateral deposited with a broker for securities sold short

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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.

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

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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.

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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.

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

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The total amount of proceeds received for the sale of securities, categorized neither as held-to-maturity nor trading securities, during the reporting period.

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

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This item represents an other than temporary decline in value that has been recognized against an investment accounted for under the cost method of accounting. The excess of the carrying amount over the fair value of the investment represents the amount of the write down which is or was reflected in earnings. The written down value is a new cost basis with the adjusted value of the investment becoming its new carrying value. Evidence of a loss in value might include, but would not necessarily be limited to, absence of an ability to recover the carrying amount of the investment or inability of the investee to sustain an earnings capacity which would justify the carrying amount of the investment.

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Amount of deferred tax liability attributable to taxable temporary differences classified as other.

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The total amount of investments that are intended to be held for an extended period of time (longer than one operating cycle).

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The reporting entity's maximum amount of exposure to loss as a result of its involvement with the Variable Interest Entity (VIE).

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Investments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
Investments (Details) - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Investments [Line Items]
Carrying Value	$ 199.6		$ 199.6		$ 377.4
Available-for-sale securities
Book cost basis	9.2		9.2		28.4
Gross unrealized gains	2.7		2.7		152.1
Market value	11.9		11.9		180.5
Proceeds from sale of investments			0.9	$ 61.2
Gross realized gains	0.1	$ 0.2	170.1	6.0
Gross realized losses		0.1	14.3	3.8
Seed capital investments cash collateral					8.4
Lower of cost or market valuation adjustments on cost method investments	0.0	$ 0.0	4.5	$ 0.0
Variable Interest Entity, Maximum Loss Exposure, Amount	96.7		96.7		115.0
Cost method, private equity and other investments
Investments [Line Items]
Carrying Value	48.4		48.4		16.8
Available-for-sale securities
Investments [Line Items]
Carrying Value	11.9		11.9		180.5
Available-for-sale securities
Deferred Tax Liabilities					62.2
Available-for-sale securities \| State Street Corporation
Investments [Line Items]
Carrying Value	0.0		0.0		169.6
Available-for-sale securities \| Other available-for-sale securities
Investments [Line Items]
Carrying Value	11.9		11.9		10.9
Other investments
Investments [Line Items]
Carrying Value	187.7		187.7		196.9
Other investments \| Trading securities
Investments [Line Items]
Carrying Value	34.1		34.1		7.9
Other investments \| Seed Capital Investments
Investments [Line Items]
Carrying Value	11.7		11.7		61.0
Other investments \| Seed Capital Investments 1
Investments [Line Items]
Carrying Value					7.4
Other investments \| Seed Capital Investments 2
Investments [Line Items]
Carrying Value					53.6
Other investments \| Cost method, private equity and other investments
Investments [Line Items]
Carrying Value	141.9		141.9		128.0
Private Equity Funds
Investments [Line Items]
Carrying Value	93.5		93.5		$ 111.2
Available-for-sale securities
Future capital commitments related to private equity fund investments	$ 3.2		$ 3.2