Allowance for Probable Loan Losses (Details 6) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Loan portfolio by credit quality indicator
Portfolio, total	$ 4,826,509	$ 4,775,004
Pass
Loan portfolio by credit quality indicator
Portfolio, total	4,470,205	4,369,031
Special Review
Loan portfolio by credit quality indicator
Portfolio, total	125,815	169,097
Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	51,638	65,855
Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	99,378	88,016
Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	79,473	83,005
Commercial
Loan portfolio by credit quality indicator
Portfolio, total	868,957	769,110
Commercial | Pass
Loan portfolio by credit quality indicator
Portfolio, total	775,851	675,263
Commercial | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	3,986	4,278
Commercial | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	15,888	16,535
Commercial | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	40,124	40,266
Commercial | Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	33,108	32,768
Commercial real estate: other construction and land development
Loan portfolio by credit quality indicator
Portfolio, total	1,085,506	1,147,669
Commercial real estate: other construction and land development | Pass
Loan portfolio by credit quality indicator
Portfolio, total	992,504	1,038,749
Commercial real estate: other construction and land development | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	34,790	55,079
Commercial real estate: other construction and land development | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	2,901	2,614
Commercial real estate: other construction and land development | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	29,139	22,567
Commercial real estate: other construction and land development | Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	26,172	28,660
Commercial real estate: farmland and commercial
Loan portfolio by credit quality indicator
Portfolio, total	1,709,437	1,673,322
Commercial real estate: farmland and commercial | Pass
Loan portfolio by credit quality indicator
Portfolio, total	1,555,921	1,486,572
Commercial real estate: farmland and commercial | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	86,445	109,144
Commercial real estate: farmland and commercial | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	32,798	46,316
Commercial real estate: farmland and commercial | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	22,373	17,345
Commercial real estate: farmland and commercial | Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	11,900	13,945
Commercial real estate: multifamily
Loan portfolio by credit quality indicator
Portfolio, total	78,854	82,948
Commercial real estate: multifamily | Pass
Loan portfolio by credit quality indicator
Portfolio, total	78,465	82,542
Commercial real estate: multifamily | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	51	53
Commercial real estate: multifamily | Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	338	353
Residential: first lien
Loan portfolio by credit quality indicator
Portfolio, total	450,391	456,731
Residential: first lien | Pass
Loan portfolio by credit quality indicator
Portfolio, total	439,754	446,218
Residential: first lien | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	517	519
Residential: first lien | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	5,574	6,338
Residential: first lien | Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	4,546	3,656
Residential: junior lien
Loan portfolio by credit quality indicator
Portfolio, total	378,697	381,736
Residential: junior lien | Pass
Loan portfolio by credit quality indicator
Portfolio, total	375,111	378,000
Residential: junior lien | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	77	77
Residential: junior lien | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total		309
Residential: junior lien | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	1,807	1,500
Residential: junior lien | Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	1,702	1,850
Consumer
Loan portfolio by credit quality indicator
Portfolio, total	70,030	74,514
Consumer | Pass
Loan portfolio by credit quality indicator
Portfolio, total	68,785	73,188
Consumer | Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	1,245	1,326
Foreign
Loan portfolio by credit quality indicator
Portfolio, total	184,637	188,974
Foreign | Pass
Loan portfolio by credit quality indicator
Portfolio, total	183,814	188,499
Foreign | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total		28
Foreign | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	361
Foreign | Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	$ 462	$ 447