Financial Instruments Owned and Pledged as Collateral at Fair Value, Securities Purchased Under Agreements to Resell and Securities Sold Under Agreements to Repurchase Summary of Assets, Liabilities and Collateral Received or Pledged (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Financial Instruments Owned and Pledged as Collateral [Line Items]
Financial Instruments Owned and Pledged as Collateral at Fair Value	$ 51,655	$ 98,688
Securities Purchased Under Agreements to Resell	351	7,669
Securities Sold Under Agreements to Repurchase	(52,025)	(106,499)
Asset (Liability) Balance [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Financial Instruments Owned and Pledged as Collateral at Fair Value	51,655	98,688
Securities Purchased Under Agreements to Resell	351	7,669
Total Assets	52,006	106,357
Securities Sold Under Agreements to Repurchase	(52,025)	(106,499)
Market Value of Collateral Received or (Pledged) [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Securities Purchased Under Agreements to Resell	352	7,671
Securities Sold Under Agreements to Repurchase	$ (52,041)	$ (106,632)