Allowance for Probable Loan Losses (Details 6) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Loan portfolio by credit quality indicator
Portfolio, total	$ 4,935,482	$ 5,053,475
Pass
Loan portfolio by credit quality indicator
Portfolio, total	4,442,864	4,496,404
Special Review
Loan portfolio by credit quality indicator
Portfolio, total	164,265	165,714
Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	63,212	59,349
Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	135,465	195,703
Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	129,676	136,305
Commercial
Loan portfolio by credit quality indicator
Portfolio, total	799,635	773,816
Commercial | Pass
Loan portfolio by credit quality indicator
Portfolio, total	686,900	655,154
Commercial | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	5,488	5,279
Commercial | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	6,140	6,361
Commercial | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	74,298	79,419
Commercial | Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	26,809	27,603
Commercial real estate: other construction and land development
Loan portfolio by credit quality indicator
Portfolio, total	1,269,146	1,273,389
Commercial real estate: other construction and land development | Pass
Loan portfolio by credit quality indicator
Portfolio, total	1,076,334	1,058,843
Commercial real estate: other construction and land development | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	75,585	76,722
Commercial real estate: other construction and land development | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	14,857	11,083
Commercial real estate: other construction and land development | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	31,870	66,313
Commercial real estate: other construction and land development | Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	70,500	60,428
Commercial real estate: farmland and commercial
Loan portfolio by credit quality indicator
Portfolio, total	1,595,721	1,664,687
Commercial real estate: farmland and commercial | Pass
Loan portfolio by credit quality indicator
Portfolio, total	1,421,917	1,449,822
Commercial real estate: farmland and commercial | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	83,062	83,581
Commercial real estate: farmland and commercial | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	41,502	40,510
Commercial real estate: farmland and commercial | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	22,676	48,543
Commercial real estate: farmland and commercial | Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	26,564	42,231
Commercial real estate: multifamily
Loan portfolio by credit quality indicator
Portfolio, total	101,245	121,599
Commercial real estate: multifamily | Pass
Loan portfolio by credit quality indicator
Portfolio, total	100,791	121,188
Commercial real estate: multifamily | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	57
Commercial real estate: multifamily | Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	397	411
Residential: first lien
Loan portfolio by credit quality indicator
Portfolio, total	476,699	495,722
Residential: first lien | Pass
Loan portfolio by credit quality indicator
Portfolio, total	467,469	490,924
Residential: first lien | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	130	132
Residential: first lien | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	309	974
Residential: first lien | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	6,561	1,402
Residential: first lien | Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	2,230	2,290
Residential: junior lien
Loan portfolio by credit quality indicator
Portfolio, total	387,488	400,148
Residential: junior lien | Pass
Loan portfolio by credit quality indicator
Portfolio, total	385,168	397,861
Residential: junior lien | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	314	319
Residential: junior lien | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	3	6
Residential: junior lien | Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	2,003	1,962
Consumer
Loan portfolio by credit quality indicator
Portfolio, total	87,660	94,109
Consumer | Pass
Loan portfolio by credit quality indicator
Portfolio, total	86,494	92,714
Consumer | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	39	41
Consumer | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total		20
Consumer | Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	1,127	1,334
Foreign
Loan portfolio by credit quality indicator
Portfolio, total	217,888	230,005
Foreign | Pass
Loan portfolio by credit quality indicator
Portfolio, total	217,791	229,898
Foreign | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	51	61
Foreign | Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	$ 46	$ 46