Amortized Cost and Estimated Fair Values of Debt Securities Held-To-Maturity by Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Amortized Cost
Due in one year or less	$ 11,939	$ 18,860
Due after one year through five years	5,605	6,937
Due after five years through ten years	80,110	98,428
Due after ten years	67,192	54,240
Total debt securities held-to-maturity	164,846	178,465
Estimated Fair Value
Due in one year or less	12,168	19,203
Due after one year through five years	5,731	7,144
Due after five years through ten years	82,340	101,008
Due after ten years	69,157	56,812
Total debt securities held-to-maturity	$ 169,396	$ 184,167