Borrowed Funds (Narrative) (Detail) (USD $)	12 Months Ended			1 Months Ended	12 Months Ended			1 Months Ended	12 Months Ended		1 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Jul. 31, 2010 Summit Trust [Member]	Dec. 31, 2012 Trust II [Member]	Dec. 31, 2004 Trust II [Member]	Dec. 31, 2011 Trust II [Member]	Feb. 28, 2007 Subordinated Notes Payable [Member]	Dec. 31, 2012 Subordinated Notes Payable [Member]	Dec. 31, 2011 Subordinated Notes Payable [Member]	Aug. 31, 2011 Subordinated Notes Payable [Member] Frost Bank [Member]	Dec. 31, 2012 Federal Funds Purchased [Member] Maximum [Member]	Dec. 31, 2012 Federal Funds Purchased [Member] Minimum [Member]	Dec. 31, 2012 Securities Sold under Agreements to Repurchase [Member] Maximum [Member]	Dec. 31, 2012 Securities Sold under Agreements to Repurchase [Member] Minimum [Member]	Dec. 31, 2012 Junior Subordinated Deferrable Interest Debenture [Member] Trust II [Member]	Dec. 31, 2011 Junior Subordinated Deferrable Interest Debenture [Member] Trust II [Member]	Dec. 31, 2012 Federal Home Loan Bank Advances [Member]	Dec. 31, 2011 Federal Home Loan Bank Advances [Member]
Borrowed Funds [Line Items]
Federal home loan bank advances																		$ 7,000	$ 26,000
Federal home loan bank advances, weighted-average rate																		6.00%	6.00%
Debt Instrument Maturity Period Days												90 days	1 day	90 days	30 days
Federal funds purchased	1,600,000	875,000
Securities sold under agreements to repurchase	559,500,000	721,300,000
Subordinated Notes								100,000,000
Subordinated Notes interest rate								5.75%			6.875%
LIBOR plus margin				Three-month LIBOR plus a margin of 2.65%		Three-month LIBOR plus a margin of 1.55%		Three-month LIBOR for the related interest period plus 0.53%	Three-month LIBOR for the related interest period plus 0.84%
Advances, variable-rate				2.65%	1.86%	1.55%	2.08%	0.53%	0.84%
Percentage of redemption price									100.00%
Unamortized debt issuance cost related to note									490,000	610,000						1,200,000	1,300,000
Maturity date								Feb. 15, 2017
Subordinated notes redemption date											August 2011
Redemption of Subordinated notes											150,000,000
Junior subordinated deferrable interest debentures	123,712,000	123,712,000		12,400,000												123,712,000
Trust preferred securities, issued					120,000,000	120,000,000
Maturity date of trust preferred securities					March 1, 2034
Common equity securities		1,360,000	11,240,000													3,700,000
Trust preferred securities, redeemed				$ 12,000,000
Maximum allowable percentage of core capital included in Tier 1 Capital by Most Bank Holding Companies	25.00%
Maximum allowable percentage of core capital included in Tier 1 Capital by Large International Bank Holding Companies	15.00%