Securities - Additional Information (Detail) (USD $)	3 Months Ended	9 Months Ended
	Sep. 30, 2012 Investment	Sep. 30, 2012 Investment	Sep. 30, 2011	Dec. 31, 2011
Schedule of Investments [Line Items]
Unrealized gains on securities available-for-sale included in accumulated other comprehensive income, net of deferred taxes	$ 16,311,000	$ 16,311,000		$ 11,128,000
Unrealized gains on securities available-for-sale included in accumulated other comprehensive income, deferred taxes	8,800,000	8,800,000		6,000,000
Investment securities with unrealized losses that are considered temporary	11,063,000	11,063,000		62,776,000
Number of investment securities which had unrealized losses for twelve months or longer	17	17
Investment securities with unrealized losses for twelve months or longer	10,402,000	10,402,000		14,787,000
Loss on investments	(14,000)	1,098,000	197,000
Other than temporary impairment losses	10,000
Security pledged as collateral, amortized cost	488,100,000	488,100,000	486,800,000
Security pledged as collateral, fair value	509,600,000	509,600,000	508,600,000
Private issue collateralized mortgage obligations
Schedule of Investments [Line Items]
Investment securities with unrealized losses that are considered temporary	10,384,000	10,384,000		10,641,000
Investment securities with unrealized losses for twelve months or longer	10,384,000	10,384,000		10,641,000
Private issue collateralized mortgage obligations | Noninvestment grade
Schedule of Investments [Line Items]
Investment securities with unrealized losses that are considered temporary	9,600,000	9,600,000
Agency Securities
Schedule of Investments [Line Items]
Number of available for sale securities sold		1
Mortgage-backed securities
Schedule of Investments [Line Items]
Investment securities with unrealized losses that are considered temporary	329,000	329,000
Investment securities with unrealized losses for twelve months or longer	$ 18,000	$ 18,000
Number of available for sale securities sold		8
Federal Home Loan Mortgage Corporation
Schedule of Investments [Line Items]
Number of preferred shares sold	587,481	587,481
Auction Rate Securities
Schedule of Investments [Line Items]
Number of available for sale securities sold		1