Gross Unrealized Losses and Fair Value by Length of Time for Held-To-Maturity Securities (Detail) (Held-to-Maturity Securities, USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Gain (Loss) on Investments [Line Items]
Number of securities	25	14
Fair Value	$ 30,401	$ 26,829
Less than 12 months	312	152
More than 12 months	0	0
Total Unrealized Losses	312	152
US Treasury and Government
Gain (Loss) on Investments [Line Items]
Number of securities	1	1
Fair Value	4,987	9,961
Less than 12 months	11	29
More than 12 months		0
Total Unrealized Losses	11	29
State and municipal
Gain (Loss) on Investments [Line Items]
Number of securities	24	13
Fair Value	25,414	16,868
Less than 12 months	301	123
More than 12 months		0
Total Unrealized Losses	$ 301	$ 123