Gross Realized Gains and Losses on All Investments (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Realized Gain Loss [Line Items]
Gross realized gains from sales of investments available-for-sale	$ 56	$ 0	$ 1,001
Gross realized losses from sales of investments available-for-sale	0	0	(371)
Net gains or (losses) from calls of investments available-for-sale	294	205	99
Net gains or (losses) from calls of investments held-to-maturity	109	87	67
Net securities gains	$ 459	$ 292	$ 796