Fair Value of Derivative Asset and Liability (Detail) (Derivatives not Designated as Hedging Instruments [Member], USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Asset Derivatives, Fair value	$ 2,870	$ 3,503
Liability Derivatives, Fair value	2,886	3,625
Interest Rate Products [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives, Fair value	2,870	3,503
Liability Derivatives, Fair value	$ 2,886	$ 3,625