Receivables and Payables (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Receivables from brokers, dealers and clearing organizations
Broker-dealers	$ 148,956,000	$ 119,268,000
Clearing organizations	1,447,000	5,725,000
Securities borrowed	496,596,000	742,307,000
Allowance for doubtful accounts	(669,000)	(1,029,000)
Total	646,330,000	866,271,000
Payables to brokers, dealers and clearing organizations
Broker-dealers	51,615,000	66,733,000
Clearing organizations	39,433,000	2,798,000
Securities loaned	508,993,000	737,789,000
Total	600,041,000	807,320,000
Receivables from customers
Customers	108,518,000	73,795,000
Allowance for doubtful accounts	(583,000)	(1,135,000)
Total	107,935,000	72,660,000
Payables to customers
Customers	11,132,000	16,797,000
Total	11,132,000	16,797,000
Allowance for doubtful accounts
Provision for doubtful accounts	(801,000)	(405,000)	1,401,000
Reduction of other general and administrative expense	1,000,000
Securities Borrowed and Loaned
Fair value of securities borrowed	481,000,000
Interest earned	15,072,000	17,756,000	19,242,000
Interest incurred	(10,218,000)	(10,786,000)	(14,589,000)
Interest earned (incurred), net	4,854,000	6,970,000	4,653,000
Gain related to adjustments to historical dividend withholdings on securities borrowed		2,500,000
Deposits paid for securities borrowed
Gross Amounts of Recognized Assets	496,596,000	742,307,000
Net Amounts Presented in the Consolidated Statement of Financial Condition	496,596,000	742,307,000
Collateral Received or Pledged (including Cash)	496,374,000	742,083,000
Net Amount	222,000	224,000
Deposits received for securities loaned
Gross Amounts of Recognized Liabilities	(508,993,000)	(737,789,000)
Net Amounts Presented in the Consolidated Statement of Financial Condition	(508,993,000)	(737,789,000)
Collateral Received or Pledged (including Cash)	(497,462,000)	(721,821,000)
Net Amount	$ (11,531,000)	$ (15,968,000)