Allowance for Probable Loan Losses (Details 6) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Loan portfolio by credit quality indicator
Portfolio, total	$ 5,286,500	$ 5,199,235
Pass
Loan portfolio by credit quality indicator
Portfolio, total	5,029,516	4,942,387
Special Review
Loan portfolio by credit quality indicator
Portfolio, total	75,336	73,077
Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	35,835	35,619
Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	69,550	72,259
Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	76,263	75,893
Commercial
Loan portfolio by credit quality indicator
Portfolio, total	1,094,563	1,042,642
Commercial | Pass
Loan portfolio by credit quality indicator
Portfolio, total	1,001,514	955,522
Commercial | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	7,213	2,270
Commercial | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	4,887	4,389
Commercial | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	45,410	46,278
Commercial | Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	35,539	34,183
Commercial real estate: other construction and land development
Loan portfolio by credit quality indicator
Portfolio, total	1,279,003	1,208,508
Commercial real estate: other construction and land development | Pass
Loan portfolio by credit quality indicator
Portfolio, total	1,242,189	1,167,295
Commercial real estate: other construction and land development | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	6,296	14,247
Commercial real estate: other construction and land development | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	8,577	9,318
Commercial real estate: other construction and land development | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	8,383	3,672
Commercial real estate: other construction and land development | Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	13,558	13,976
Commercial real estate: farmland and commercial
Loan portfolio by credit quality indicator
Portfolio, total	1,721,923	1,750,039
Commercial real estate: farmland and commercial | Pass
Loan portfolio by credit quality indicator
Portfolio, total	1,608,212	1,635,179
Commercial real estate: farmland and commercial | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	61,709	56,438
Commercial real estate: farmland and commercial | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	22,371	21,912
Commercial real estate: farmland and commercial | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	14,024	20,472
Commercial real estate: farmland and commercial | Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	15,607	16,038
Commercial real estate: multifamily
Loan portfolio by credit quality indicator
Portfolio, total	108,106	102,098
Commercial real estate: multifamily | Pass
Loan portfolio by credit quality indicator
Portfolio, total	106,984	100,948
Commercial real estate: multifamily | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	842	855
Commercial real estate: multifamily | Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	280	295
Residential: first lien
Loan portfolio by credit quality indicator
Portfolio, total	421,009	438,462
Residential: first lien | Pass
Loan portfolio by credit quality indicator
Portfolio, total	414,523	432,067
Residential: first lien | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	118	122
Residential: first lien | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	95	120
Residential: first lien | Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	6,273	6,153
Residential: junior lien
Loan portfolio by credit quality indicator
Portfolio, total	419,156	409,230
Residential: junior lien | Pass
Loan portfolio by credit quality indicator
Portfolio, total	415,575	405,731
Residential: junior lien | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	458	293
Residential: junior lien | Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	3,123	3,206
Consumer
Loan portfolio by credit quality indicator
Portfolio, total	63,430	66,414
Consumer | Pass
Loan portfolio by credit quality indicator
Portfolio, total	61,977	64,808
Consumer | Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	1,453	1,606
Foreign
Loan portfolio by credit quality indicator
Portfolio, total	179,310	181,842
Foreign | Pass
Loan portfolio by credit quality indicator
Portfolio, total	178,542	180,837
Foreign | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	338	569
Foreign | Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	$ 430	$ 436