Amortized Cost and Estimated Fair Values of Debt Securities Held-To-Maturity by Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Amortized Cost
Due in one year or less	$ 7,431	$ 18,860
Due after one year through five years	4,653	6,937
Due after five years through ten years	116,735	98,428
Due after ten years	86,995	54,240
Total debt securities held-to-maturity	215,814	178,465
Estimated Fair Value
Due in one year or less	7,523	19,203
Due after one year through five years	4,725	7,144
Due after five years through ten years	120,074	101,008
Due after ten years	89,702	56,812
Total debt securities held-to-maturity	$ 222,024	$ 184,167