Investment Securities (Narrative) (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2012 Security	Dec. 31, 2011	Sep. 30, 2011
Investment [Line Items]
Amount of securities pledged to secure public funds and other purposes as required by law	$ 1,930,861	$ 1,673,419
Number of non-agency residential mortgage-backed securities that had unrealized losses	2
Number of non-agency residential-mortgage backed securities with other-than-temporary impairment	1
Non-credit related unrealized loss on other-than-temporarily impaired debt securities, net of taxes	1,096		1,996
Unrealized loss	21,174	26,314
Other Structured Financial Products [Member]
Investment [Line Items]
Unrealized loss	$ 16,577	$ 11,538	$ 12,052
Minimum [Member]
Investment [Line Items]
Number of days mortgage is past due related to cumulative default rate	60
Maximum [Member]
Investment [Line Items]
Number of days mortgage is past due related to cumulative default rate	90