Investment Securities - Realized Gains and Losses on Sales of Investment Securities (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Gross gains, Sales transactions									$ 6,621	$ 31,456	$ 8,615
Gross losses, Sales transactions									(1,050)		(96)
Gains on Sales transactions, net									5,571	31,456	8,519
Gross gains, Maturities and other securities transactions									2,327	623	3,158
Gross losses, Maturities and other securities transactions									(5,311)	(11)	(79)
Gains (losses) on Maturities and other securities transactions, net									(2,984)	612	3,079
Gains on securities transactions, net	$ 44	$ 1,496	$ 1,204	$ (157)	$ 12,034	$ 863	$ 16,492	$ 2,679	$ 2,587	$ 32,068	$ 11,598