Derivative Financial Instruments Derivative Financial Instruments (Derivative Asset and Liability Balance Sheet Offsetting) (Detail) (USD $)	Mar. 31, 2015		Dec. 31, 2014		Mar. 31, 2014
Derivative Assets
Gross Amounts Recognized	$ 47,550,000		$ 37,841,000		$ 37,239,000
Less: Amounts offset in the Statements of Financial Condition	0		0		0
Net amount presented in the Statements of Financial Condition	47,550,000		37,841,000		37,239,000
Offsetting Derivative Positions	(1,563,000)		(2,771,000)		(7,359,000)
Collateral Posted	0		0		0
Net Credit Exposure	45,987,000		35,070,000		29,880,000
Derivative Liabilities
Gross Amounts Recognized	47,698,000		36,921,000		34,990,000
Less: Amounts offset in the Statements of Financial Condition	0		0		0
Net amount presented in the Statements of Financial Condition	47,698,000		36,921,000		34,990,000
Offsetting Derivative Positions	(1,563,000)		(2,771,000)		(7,359,000)
Collateral Posted	(46,135,000)	[1]	(34,150,000)	[1]	(27,631,000)	[1]
Net Credit Exposure	0		0		0
Security Owned and Pledged as Collateral, Fair Value	$ 51,300,000		$ 43,800,000		$ 37,100,000

[1] As of March 31, 2015, December 31, 2014 and March 31, 2014, the Company posted collateral of $51.3 million, $43.8 million and $37.1 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.