Fair Value of Financial Assets and Liabilities - Offsetting of Derivative Instruments and Collateralized Transactions (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross Amounts	$ 71,282	$ 1,781
Gross Amounts Offset	241	999
Net Amounts Presented in the Statements of Financial Condition	71,041	782
Gross Amounts Not Offset in the Statements of Financial Condition Related to Financial Instruments
Gross Amounts Not Offset in the Statements of Financial Condition related to Cash Collateral Received
Net Amounts	71,041	782
Gross Amounts	1,713	1,337
Gross Amounts Offset	241	999
Net Amounts Presented in the Statements of Financial Condition	1,472	338
Gross Amounts Not Offset in the Statements of Financial Condition Related to Financial Instruments
Gross Amounts Not Offset in the Statements of Financial Condition related to Cash Collateral Received
Net Amounts	1,472	338
Interest rate swaps [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross Amounts	671	990
Gross Amounts Offset	204	208
Net Amounts Presented in the Statements of Financial Condition	467	782
Gross Amounts Not Offset in the Statements of Financial Condition Related to Financial Instruments
Gross Amounts Not Offset in the Statements of Financial Condition related to Cash Collateral Received
Net Amounts	467	782
Gross Amounts	204	208
Gross Amounts Offset	204	208
Net Amounts Presented in the Statements of Financial Condition
Gross Amounts Not Offset in the Statements of Financial Condition Related to Financial Instruments
Gross Amounts Not Offset in the Statements of Financial Condition related to Cash Collateral Received
Net Amounts
Foreign exchange swaps [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross Amounts	171	791
Gross Amounts Offset	37	791
Net Amounts Presented in the Statements of Financial Condition	134
Gross Amounts Not Offset in the Statements of Financial Condition Related to Financial Instruments
Gross Amounts Not Offset in the Statements of Financial Condition related to Cash Collateral Received
Net Amounts	134
Gross Amounts	1,509	1,129
Gross Amounts Offset	37	791
Net Amounts Presented in the Statements of Financial Condition	1,472	338
Gross Amounts Not Offset in the Statements of Financial Condition Related to Financial Instruments
Gross Amounts Not Offset in the Statements of Financial Condition related to Cash Collateral Received
Net Amounts	1,472	338
Securities borrowed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross Amounts	70,440
Gross Amounts Offset
Net Amounts Presented in the Statements of Financial Condition	70,440
Gross Amounts Not Offset in the Statements of Financial Condition Related to Financial Instruments
Gross Amounts Not Offset in the Statements of Financial Condition related to Cash Collateral Received
Net Amounts	$ 70,440