Summary of Significant Accounting Policies - Estimated Fair Values of Financial Instrument Assets and Liabilities Which are Not Measured at Fair Value on the Consolidated Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Carrying Amount [Member]
Financial Assets
Cash and cash equivalents	$ 70,067	$ 108,962
Due from broker, dealers and clearing organizations	5,513	94,489
Non-interest bearing receivables	7,324	8,576
Other investments, at cost	5,000	7,000
Financial Liabilities
Accounts payable, accrued expenses and other liabilities	18,680	23,093
Estimated Fair Value [Member]
Financial Assets
Cash and cash equivalents	70,067	108,962
Due from broker, dealers and clearing organizations	5,513	94,489
Non-interest bearing receivables	7,324	8,576
Other investments, at cost	5,000	7,395
Financial Liabilities
Accounts payable, accrued expenses and other liabilities	18,680	23,093
Estimated Fair Value [Member] | Estimated Fair Value, Level 1 [Member]
Financial Assets
Cash and cash equivalents	70,067	108,962
Due from broker, dealers and clearing organizations	0	0
Non-interest bearing receivables	0	0
Other investments, at cost	0	0
Financial Liabilities
Accounts payable, accrued expenses and other liabilities	0	0
Estimated Fair Value [Member] | Estimated Fair Value, Level 2 [Member]
Financial Assets
Cash and cash equivalents	0	0
Due from broker, dealers and clearing organizations	5,513	94,489
Non-interest bearing receivables	7,324	8,576
Other investments, at cost	0	0
Financial Liabilities
Accounts payable, accrued expenses and other liabilities	18,680	23,093
Estimated Fair Value [Member] | Estimated Fair Value, Level 3 [Member]
Financial Assets
Cash and cash equivalents	0	0
Due from broker, dealers and clearing organizations	0	0
Non-interest bearing receivables	0	0
Other investments, at cost	5,000	7,395
Financial Liabilities
Accounts payable, accrued expenses and other liabilities	$ 0	$ 0