Carrying Value and Fair Value and Fair Value Hierarchy Category of Financial Instruments Not Measured at Fair Value in Condensed Consolidated Statements of Financial Condition (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Financial Assets:
Receivables from banks and brokers	$ 83,917	$ 85,401
Level 1
Financial Assets:
Cash and cash equivalents	407,306
Level 2
Financial Assets:
Receivables from banks and brokers	83,700
Financial Liabilities:
Payables to brokers, dealers, FCMs and other regulated entities	12,554
Payables to customers	414,011
Level 3
Financial Assets:
Investment in Kapitall, Inc.	500
Financial Liabilities:
Note payable to dbFX	2,730
Carrying Value
Financial Assets:
Cash and cash equivalents	407,306
Receivables from banks and brokers	83,700
Investment in Kapitall, Inc.	500
Financial Liabilities:
Payables to brokers, dealers, FCMs and other regulated entities	12,554
Payables to customers	414,011
Note payable to dbFX	2,730
Fair Value
Financial Assets:
Cash and cash equivalents	407,306
Receivables from banks and brokers	83,700
Investment in Kapitall, Inc.	500
Financial Liabilities:
Payables to brokers, dealers, FCMs and other regulated entities	12,554
Payables to customers	414,011
Note payable to dbFX	$ 2,730