Summary of Assets, Liabilities and Collateral Received or Pledged (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Financial Instruments Owned and Pledged as Collateral [Line Items]
Financial Instruments Owned and Pledged as Collateral at Fair Value	$ 78,887	$ 120,594
Securities Purchased Under Agreements to Resell	3,879	0
Securities Sold Under Agreements to Repurchase	(82,930)	(120,787)
Asset (Liability) Balance [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Financial Instruments Owned and Pledged as Collateral at Fair Value	78,887	120,594
Securities Purchased Under Agreements to Resell	3,879	0
Total Assets	82,766	120,594
Securities Sold Under Agreements to Repurchase	(82,930)	(120,787)
Market Value Of Collateral Received Or (Pledged) [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Securities Purchased Under Agreements to Resell	3,900	0
Securities Sold Under Agreements to Repurchase	$ (83,060)	$ (121,029)