Investment Securities (Narrative) (Detail) (USD $)	6 Months Ended	12 Months Ended
	Jun. 30, 2012	Dec. 31, 2011
Schedule of Securities Available-for-sale And Held To Maturity [Line Items]
Pledged securities, carrying values	$ 1,732,462,000	$ 1,698,943,000
Number of debt securities, held	40	50
Unrealized losses on debt securities, percent of amortized cost	0.40%	0.50%
Unrealized losses on debt securities, percent of Company's total amortized cost	0.05%	0.10%
Number of debt securities having unrealized losses	40	50
Number of securities recorded with an aggregate amortized cost	7	12
Number of securities, continuous unrealized loss position, twelve months or longer, fair value	7	12
Available-for-sale securities, amortized cost basis	14,627,000	24,453,000
Unrealized loss on securities	$ 322,000	$ 391,000
Unrated revenue municipal bond	1
Impairment of par value bond, percentage	52.00%
Fannie Mae And Freddie Mac Securities Rated AAA [Member]
Schedule of Securities Available-for-sale And Held To Maturity [Line Items]
Number of securities issued	6	12