Derivative Financial Instruments (Derivative Asset and Liability Balance Sheet Offsetting) (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012		Sep. 30, 2012
Derivative Assets
Gross Amounts Recognized	$ 41,945		$ 47,546		$ 50,349
Less: Amounts offset in the Statements of Financial Condition	0		0		0
Net amount presented in the Statements of Financial Condition	41,945		47,546		50,349
Offsetting Derivative Positions	(6,362)		(339)		(258)
Securities Collateral Posted	0	[1]	0	[1]	0	[1]
Cash Collateral Posted	0		0		0
Net Credit Exposure	35,583		47,207		50,091
Derivative Liabilities
Gross Amounts Recognized	38,804		53,755		58,008
Less: Amounts offset in the Statements of Financial Condition	0		0		0
Net amount presented in the Statements of Financial Condition	38,804		53,755		58,008
Offsetting Derivative Positions	(6,362)		(339)		(258)
Securities Collateral Posted	(28,620)	[1]	(46,811)	[1]	(48,735)	[1]
Cash Collateral Posted	0		(6,605)		(7,138)
Net Credit Exposure	$ 3,822		$ 0		$ 1,877

[1] As of December 31, 2012, the Company posted securities collateral of $49.9 million which resulted in excess collateral with its counterparties. For purposes of this disclosure, the amount of posted collateral is limited to the amount offsetting the derivative liability.