Derivative Financial Instruments Derivative Financial Instruments (Derivative Asset and Liability Balance Sheet Offsetting) (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014		Sep. 30, 2014
Derivative Assets
Gross Amounts Recognized	$ 56,938	$ 37,841		$ 33,275
Less: Amounts offset in the Statements of Financial Condition	0	0		0
Net amount presented in the Statements of Financial Condition	56,938	37,841		33,275
Offsetting Derivative Positions	(614)	(2,771)		(5,417)
Collateral Posted	0	0		0
Net Credit Exposure	56,324	35,070		27,858
Derivative Liabilities
Gross Amounts Recognized	57,429	36,921		31,451
Less: Amounts offset in the Statements of Financial Condition	0	0		0
Net amount presented in the Statements of Financial Condition	57,429	36,921		31,451
Offsetting Derivative Positions	(614)	(2,771)		(5,417)
Collateral Posted	(54,410)	(34,150)	[1]	(26,034)	[1]
Net Credit Exposure	$ 2,405	0		0
Security Owned and Pledged as Collateral, Fair Value		$ 43,800		$ 33,900

[1] As of December 31, 2014 and September 30, 2014, the Company posted collateral of $43.8 million and $33.9 million, respectively, 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.