Amortized Cost and Estimated Fair Values of Debt Securities Held-To-Maturity by Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Amortized Cost
Due in one year or less	$ 5,857	$ 7,431
Due after one year through five years	2,720	4,653
Due after five years through ten years	124,771	116,735
Due after ten years	78,028	86,995
Total debt securities held-to-maturity	211,376	215,814
Estimated Fair Value
Due in one year or less	5,924	7,523
Due after one year through five years	2,731	4,725
Due after five years through ten years	128,382	120,074
Due after ten years	79,383	89,702
Total debt securities held-to-maturity	$ 216,420	$ 222,024