Derivative Financial Instruments Derivative Financial Instruments (Derivative Asset and Liability Balance Sheet Offsetting) (Detail) - USD ($) $ in Thousands		Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Derivative Assets
Gross Amounts Recognized		$ 36,747	$ 37,841	$ 37,461
Less: Amounts offset in the Statements of Financial Condition		0	0	0
Net amount presented in the Statements of Financial Condition		36,747	37,841	37,461
Offsetting Derivative Positions		(1,896)	(2,771)	(3,738)
Collateral Posted		0	0	0
Net Credit Exposure		34,851	35,070	33,723
Derivative Liabilities
Gross Amounts Recognized		36,605	36,921	36,733
Less: Amounts offset in the Statements of Financial Condition		0	0	0
Net amount presented in the Statements of Financial Condition		36,605	36,921	36,733
Offsetting Derivative Positions		(1,896)	(2,771)	(3,738)
Collateral Posted	[1]	(34,709)	(34,150)	(26,354)
Net Credit Exposure		0	0	$ 6,641
Security Owned and Pledged as Collateral, Fair Value		$ 36,000	$ 43,800

[1] As of June 30, 2015 and December 31, 2014, the Company posted collateral of $36.0 million and $43.8 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.