Allowance for Probable Loan Losses (Details 6) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Loan portfolio by credit quality indicator
Portfolio, total	$ 5,052,161	$ 4,775,004
Pass
Loan portfolio by credit quality indicator
Portfolio, total	4,688,096	4,369,031
Special Review
Loan portfolio by credit quality indicator
Portfolio, total	137,181	169,097
Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	45,485	65,855
Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	82,373	88,016
Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	99,026	83,005
Commercial
Loan portfolio by credit quality indicator
Portfolio, total	1,018,154	769,110
Commercial | Pass
Loan portfolio by credit quality indicator
Portfolio, total	911,393	675,263
Commercial | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	8,244	4,278
Commercial | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	16,623	16,535
Commercial | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	46,663	40,266
Commercial | Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	35,231	32,768
Commercial real estate: other construction and land development
Loan portfolio by credit quality indicator
Portfolio, total	1,119,342	1,147,669
Commercial real estate: other construction and land development | Pass
Loan portfolio by credit quality indicator
Portfolio, total	1,044,439	1,038,749
Commercial real estate: other construction and land development | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	25,064	55,079
Commercial real estate: other construction and land development | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	9,164	2,614
Commercial real estate: other construction and land development | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	4,371	22,567
Commercial real estate: other construction and land development | Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	36,304	28,660
Commercial real estate: farmland and commercial
Loan portfolio by credit quality indicator
Portfolio, total	1,722,956	1,673,322
Commercial real estate: farmland and commercial | Pass
Loan portfolio by credit quality indicator
Portfolio, total	1,558,965	1,486,572
Commercial real estate: farmland and commercial | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	101,674	109,144
Commercial real estate: farmland and commercial | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	19,698	46,316
Commercial real estate: farmland and commercial | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	25,581	17,345
Commercial real estate: farmland and commercial | Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	17,038	13,945
Commercial real estate: multifamily
Loan portfolio by credit quality indicator
Portfolio, total	99,912	82,948
Commercial real estate: multifamily | Pass
Loan portfolio by credit quality indicator
Portfolio, total	99,598	82,542
Commercial real estate: multifamily | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total		53
Commercial real estate: multifamily | Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	314	353
Residential: first lien
Loan portfolio by credit quality indicator
Portfolio, total	443,889	456,731
Residential: first lien | Pass
Loan portfolio by credit quality indicator
Portfolio, total	433,944	446,218
Residential: first lien | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	119	519
Residential: first lien | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	4,842	6,338
Residential: first lien | Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	4,984	3,656
Residential: junior lien
Loan portfolio by credit quality indicator
Portfolio, total	393,437	381,736
Residential: junior lien | Pass
Loan portfolio by credit quality indicator
Portfolio, total	389,920	378,000
Residential: junior lien | Special Review
Loan portfolio by credit quality indicator
Portfolio, total		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	298	1,500
Residential: junior lien | Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	3,219	1,850
Consumer
Loan portfolio by credit quality indicator
Portfolio, total	67,422	74,514
Consumer | Pass
Loan portfolio by credit quality indicator
Portfolio, total	65,937	73,188
Consumer | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	4
Consumer | Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	1,481	1,326
Foreign
Loan portfolio by credit quality indicator
Portfolio, total	187,049	188,974
Foreign | Pass
Loan portfolio by credit quality indicator
Portfolio, total	183,900	188,499
Foreign | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	2,080
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	614
Foreign | Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	$ 455	$ 447