Derivative Financial Instruments (Derivative Asset and Liability Balance Sheet Offsetting) (Detail) - USD ($) $ in Thousands		Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Derivative Assets
Gross Amounts Recognized		$ 73,985	$ 42,779	$ 47,550
Less: Amounts offset in the Statements of Financial Condition		0	0	0
Net amount presented in the Statements of Financial Condition		73,985	42,779	47,550
Offsetting Derivative Positions		(156)	(753)	(1,563)
Collateral Posted		0	0	0
Net Credit Exposure		73,829	42,026	45,987
Derivative Liabilities
Gross Amounts Recognized		74,019	42,458	47,698
Less: Amounts offset in the Statements of Financial Condition		0	0	0
Net amount presented in the Statements of Financial Condition		74,019	42,458	47,698
Offsetting Derivative Positions		(156)	(753)	(1,563)
Collateral Posted	[1]	(73,863)	(41,705)	(46,135)
Net Credit Exposure		0	0	0
Security Owned and Pledged as Collateral, Fair Value		$ 81,600	$ 45,500	$ 51,300

[1] As of March 31, 2016, December 31, 2015 and March 31, 2015, the Company posted collateral of $81.6 million, $45.5 million and $51.3 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.