Investments - Narrative (Details) (USD $)	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013	Dec. 31, 2012
Fair Value Disclosures [Abstract]
Securities valued based on dealer quotes or other quoted market prices for similar securities	$ 445,400,000	$ 483,200,000		$ 248,400,000
Percent of securities valued based on dealer quotes or other quoted market prices for similar securities	25.40%	33.10%		26.50%
Securities sold, not yet purchased, valued based on dealer quotes or other quoted market prices for similar securities	1,500,000	41,000,000		68,800,000
Percent of securities sold, not yet purchased, valued based on dealer quotes or other quoted market prices for similar securities	2.60%	73.10%		39.00%
Private common equity securities	2,800,000	3,300,000		2,800,000
Securities fair valued by using information obtained from the Company's Investment Manager representing less than 1% of total investments in securities and commodities	1.00%	1.00%		1.00%
Asset transfers from level 1 to level 2	0	0	0	0
Asset transfers from level 2 to level 1	86.6	0	0	0
Liability transfers from level 1 to level 2	0	0	0	0
Liability transfers from level 2 to level 1	0	0	0	0
Total unrealized gain (loss) related to fair value assets using significant unobservable inputs (Level 3	$ 400,000	$ 1,000,000	$ 1,800,000	$ (700,000)

Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Assets, Unrealized Gain (Loss)

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Financial Instruments, Owned, Private Securities, at Fair Value

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Private Securities, Percent of Investment in Securities and Commodities

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Security Owned and Sold Not Yet Purchased, Not Readily Marketable, Quoted Price, Percent

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Security Owned Not Readily Marketable, Quoted Price, Percent

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Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

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Amount of transfers of assets measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1.

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Amount of transfers of liabilities measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

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Amount of transfers of liabilities measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1.

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- Definition

Fair value of financial instruments sold short by a broker-dealer for their own account (proprietary securities) for trading or investment purposes determined by management based upon quoted prices of a financial instrument with similar characteristics or on a valuation technique or model.

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

-Publisher FASB

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-Topic 825

-SubTopic 10

-Section 50

-Paragraph 10

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Amount of not readily marketable security or investment measured by quoted price.

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