Amortized Cost and Estimated Fair Values of Securities Available for Sale by Contractual Maturities and Historical Prepayments Based on Prior Twelve Months of Principal Payments (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Investments Debt And Equity Securities [Abstract]
Securities available for sale, amortized cost, maturing in one year or less	$ 11,443	$ 11,679
Securities available for sale, estimated fair value, maturing in one year or less	11,113	11,788
Weighted average yield, maturing in one year or less	1.28%	1.59%
Securities available for sale, amortized cost, maturing over one year through five years	32,600	35,038
Securities available for sale, estimated fair value, maturing over one year through five years	31,723	35,264
Weighted average yield, maturing over one year through five years	1.51%	1.62%
Securities available for sale, amortized cost, maturing over five years through ten years	17,803	24,738
Securities available for sale, estimated fair value, maturing over five years through ten years	17,096	24,747
Weighted average yield, maturing over five years through ten years	1.49%	1.62%
Securities available for sale, amortized cost, maturing over ten years	11,826	15,851
Securities available for sale, estimated fair value, maturing over ten years	10,018	14,579
Weighted average yield, maturing over ten years	1.36%	1.43%
Securities available for sale, amortized cost, total	73,672	87,306
Securities available for sale, estimated fair value, total	$ 69,950	$ 86,378
Weighted average yield	1.45%	1.58%