Collateralized Transactions - Narrative (Details)		3 Months Ended			6 Months Ended
	Sep. 26, 2013 USD ($)	Jun. 30, 2015 USD ($) broker-dealer	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Jun. 30, 2015 USD ($) broker-dealer	Jun. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Bank call loans		$ 177,500,000			$ 177,500,000		$ 59,400,000
Customer securities under customer margin loans available to be pledged		1,300,000,000			1,300,000,000
Customer securities under customer margin loans agreement available to be repledged		156,400,000			156,400,000
Customer securities deposited to secure obligations and margin requirements under option contracts		483,700,000			483,700,000
Outstanding letters of credit		0			0
Fair value of the reverse repurchase agreements		104,301,000			104,301,000		250,000,000
Carrying value of pledged securities owned that can be sold or re-pledged by the counterparty		704,200,000			704,200,000		518,100,000
Carrying value of securities owned by the Company loaned or pledged		197,900,000			$ 197,900,000		149,100,000
Amount of first interim distribution	$ 9,500,000	$ 437,000		$ 600,000
Receivable from brokers and clearing organizations, number of major broker-dealers | broker-dealer		2			2
Receivable from brokers and clearing organizations		$ 124,100,000			$ 124,100,000
Credit Concentration Risk [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Guaranteed mortgages for a period					15 days
Variable interest rate under warehouse facility					1 month LIBOR
Interest expense		385,000	$ 74,800		$ 510,000	$ 225,400
Warehouse Facility [Member] | Credit Concentration Risk [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Outstanding under the warehouse facility line		70,900,000			70,900,000
Reverse Repurchase Agreements [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Customer securities under customer margin loans agreement available to be repledged		365,100,000			365,100,000		314,100,000
Securities received as collateral under securities borrowed transaction with market value		365,700,000			365,700,000		312,600,000
Securities Borrowed Transactions [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Customer securities under customer margin loans agreement available to be repledged		43,100,000			43,100,000		4,400,000
Securities received as collateral under securities borrowed transaction with market value		272,900,000			272,900,000		$ 235,100,000
Corporate [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Collateralized loans, collateralized by firm		198,200,000			198,200,000
Other Than Corporate Customer [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Collateralized loans, collateralized by customer securities		$ 246,000,000			$ 246,000,000