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, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Financial Instruments Owned and Pledged as Collateral [Line Items]
Financial Instruments Owned and Pledged as Collateral at Fair Value	$ 84,866	$ 120,594
Securities Purchased Under Agreements to Resell	7,075	0
Securities Sold Under Agreements to Repurchase	(92,110)	(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	84,866	120,594
Securities Purchased Under Agreements to Resell	7,075	0
Total Assets	91,941	120,594
Securities Sold Under Agreements to Repurchase	(92,110)	(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	7,070	0
Securities Sold Under Agreements to Repurchase	$ (92,282)	$ (121,029)