Investment Securities (Narrative) (Detail) (USD $)	3 Months Ended		9 Months Ended			9 Months Ended			9 Months Ended		9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011	Sep. 30, 2012 Other Non Interest Income [Member]	Sep. 30, 2012 Pooled Trust Preferred Securities [Member]	Dec. 31, 2011 Pooled Trust Preferred Securities [Member]	Sep. 30, 2012 Single Issuer Trust Preferred Securities [Member] Location Investment	Dec. 31, 2011 Single Issuer Trust Preferred Securities [Member]	Sep. 30, 2012 Agency CMOs - GSE [Member]	Dec. 31, 2011 Agency CMOs - GSE [Member]	Sep. 30, 2012 Equity Securities [Member]	Dec. 31, 2011 Equity Securities [Member]	Sep. 30, 2012 Mortgage-backed Securities, Issued by Private Enterprises [Member]	Dec. 31, 2011 Mortgage-backed Securities, Issued by Private Enterprises [Member]	Sep. 30, 2012 Mortgage-Backed Securities [Member]	Dec. 31, 2011 Mortgage-Backed Securities [Member]	Sep. 30, 2012 Mortgage-Backed Securities [Member] Categories of Investments, Marketable Securities, Held-to-maturity Securities [Member]	Sep. 30, 2012 Municipal Bonds And Notes [Member]	Dec. 31, 2011 Municipal Bonds And Notes [Member]
Schedule of Investments [Line Items]
Callable securities			$ 589,300,000
Pledged securities as required or permitted by law	2,500,000,000		2,500,000,000		2,400,000,000
Percent of government obligations to consolidated shareholders' equity maximum	10.00%		10.00%		10.00%
Total investment securities - Fair value - Total	312,729,000		312,729,000		662,741,000		28,300,000	29,000,000	41,400,000	38,200,000
Gross unrealized loss							21,300,000		9,700,000		275,000	3,300,000	20,000	24,000	3,900,000	11,200,000	628,000	158,000	37,000	178,000	174,000
Decrease in pooled issuers trust preferred securities							700,000
Increase in trust preferred securities - single issuers									3,200,000
Number of investments issued									5
Number of large capitalization money center financial institutions									3
Percentage of bank qualified bonds with credit rating range of AAA to A			94.50%
Percentage of municipal bond Portfolio comprised of General Obligation bonds			85.80%
Percentage of municipal bond portfolio comprised of Revenue bonds			13.90%
Percentage of municipal bond portfolio comprised of other bonds			0.003
Private equity fund investments, fair value	11,900,000		11,900,000
Realized Loss on private equity funds, net of OTTI charges	478,000		641,000			665,000
Realized Gain on private equity funds, net of OTTI charges		91,000		1,500,000
Held-to-maturity Securities, Unrecognized Holding Loss	$ 291,000		$ 291,000		$ 174,000												$ 37,000			$ 178,000	$ 174,000