Amortized Cost and Estimated Fair Values of Debt Securities Held-To-Maturity by Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Amortized Cost
Due in one year or less	$ 10,433	$ 18,860
Due after one year through five years	5,026	6,937
Due after five years through ten years	105,709	98,428
Due after ten years	85,445	54,240
Total debt securities held-to-maturity	206,613	178,465
Estimated Fair Value
Due in one year or less	10,635	19,203
Due after one year through five years	5,122	7,144
Due after five years through ten years	108,745	101,008
Due after ten years	88,733	56,812
Total debt securities held-to-maturity	$ 213,235	$ 184,167