Collateralized Transactions - Additional Information (Detail) (USD $)	0 Months Ended				3 Months Ended		6 Months Ended
	Sep. 26, 2013	Jun. 30, 2014 broker-dealer	Mar. 31, 2014	Dec. 31, 2013	Jun. 30, 2014 Credit Concentration Risk [Member]	Jun. 30, 2013 Credit Concentration Risk [Member]	Jun. 30, 2014 Credit Concentration Risk [Member]	Jun. 30, 2013 Credit Concentration Risk [Member]	Jun. 30, 2014 Warehouse Facility [Member] Credit Concentration Risk [Member]	Jun. 30, 2014 Reverse Repurchase Agreements [Member]	Dec. 31, 2013 Reverse Repurchase Agreements [Member]	Jun. 30, 2014 Securities Borrowed Transactions [Member]	Dec. 31, 2013 Securities Borrowed Transactions [Member]	Jun. 30, 2014 Corporate [Member]	Jun. 30, 2014 Other Than Corporate Customer [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Bank call loans		$ 147,200,000		$ 118,200,000
Collateralized loans, collateralized by firm														105,000,000
Collateralized loans, collateralized by customer securities															258,500,000
Customer securities under customer margin loans available to be pledged		1,400,000,000
Customer securities under customer margin loans agreement available to be repledged		169,500,000								314,800,000	385,500,000	27,200,000	11,000,000
Customer securities deposited to secure obligations and margin requirements under option contracts		349,200,000
Outstanding letters of credit		0
Fair value of the reverse repurchase agreements		250,000,000		184,000,000
Securities received as collateral under securities borrowed transaction with market value										314,800,000	385,500,000	239,100,000	265,300,000
Carrying value of pledged securities owned that can be sold or re-pledged by the counterparty		664,700,000		586,600,000
Carrying value of securities owned by the Company loaned or pledged		125,900,000		126,800,000
Amount of first interim distribution	9,500,000
Fair value of the company's investment on reserves not paid in first distribution			600,000
Receivable from brokers and clearing organizations, number of major broker-dealers		3
Receivable from brokers and clearing organizations		157,800,000
Guaranteed mortgages for a period							15 days
Outstanding under the warehouse facility line									14,300,000
Variable interest rate under warehouse facility							1 month LIBOR
Interest expense					$ 74,800	$ 149,200	$ 225,400	$ 291,300