Realized Gains and Losses on Sales of Investment Securities (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Gross gains, Sales transactions	$ 4,513	$ 450	$ 5,887	$ 19,418
Gross losses, Sales transactions			(298)
Gains on Sales transactions, net	4,513	450	5,589	19,418
Gross gains, Maturities and other securities transactions	2,242	414	2,261	622
Gross losses, Maturities and other securities transactions	(5,259)	(1)	(5,307)	(6)
Gains (losses) on Maturities and other securities transactions, net	(3,017)	413	(3,046)	616
Total (losses) gains on securities transactions, net	$ 1,496	$ 863	$ 2,543	$ 20,034