Effect of Master Netting Arrangements on Presentation of Derivative Assets and Liabilities in Consolidated Statements of Financial Condition (Detail) (Derivatives, net, USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Derivative Financial Instruments, Assets
Effect of derivatives on statement of operations and comprehensive income [Line Items]
Gross Amounts of Recognized Assets	$ 52,056	$ 30,295
Gross Amounts offset in the statement of financial position	35,988	4,730
Net Amounts of Assets Presented in the Statement of Financial Position	16,068	25,565
Financial Instruments
Cash Collateral Received		41
Net Amount	16,068	25,524
Derivative Financial Instruments, Liabilities
Effect of derivatives on statement of operations and comprehensive income [Line Items]
Gross Amounts of Recognized Liabilities	32,744	8,111
Gross Amounts Offset in the Statement of Financial Position	25,810	4,730
Net Amounts of Liabilities Presented in the Statement of Financial Position	6,934	3,381
Financial Instruments
Cash Collateral Received		2,795
Net Amount	$ 6,934	$ 586