Allowance for Probable Loan Losses (Details 6) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Loan portfolio by credit quality indicator
Portfolio, total	$ 4,878,322	$ 5,053,475
Pass
Loan portfolio by credit quality indicator
Portfolio, total	4,498,682	4,496,404
Special Review
Loan portfolio by credit quality indicator
Portfolio, total	104,677	165,714
Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	55,749	59,349
Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	140,949	195,703
Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	78,265	136,305
Commercial
Loan portfolio by credit quality indicator
Portfolio, total	808,241	773,816
Commercial | Pass
Loan portfolio by credit quality indicator
Portfolio, total	691,662	655,154
Commercial | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	5,460	5,279
Commercial | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	5,522	6,361
Commercial | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	77,544	79,419
Commercial | Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	28,053	27,603
Commercial real estate: other construction and land development
Loan portfolio by credit quality indicator
Portfolio, total	1,210,491	1,273,389
Commercial real estate: other construction and land development | Pass
Loan portfolio by credit quality indicator
Portfolio, total	1,131,653	1,058,843
Commercial real estate: other construction and land development | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	14,641	76,722
Commercial real estate: other construction and land development | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	5,411	11,083
Commercial real estate: other construction and land development | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	33,070	66,313
Commercial real estate: other construction and land development | Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	25,716	60,428
Commercial real estate: farmland and commercial
Loan portfolio by credit quality indicator
Portfolio, total	1,623,053	1,664,687
Commercial real estate: farmland and commercial | Pass
Loan portfolio by credit quality indicator
Portfolio, total	1,453,930	1,449,822
Commercial real estate: farmland and commercial | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	84,131	83,581
Commercial real estate: farmland and commercial | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	44,381	40,510
Commercial real estate: farmland and commercial | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	22,283	48,543
Commercial real estate: farmland and commercial | Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	18,328	42,231
Commercial real estate: multifamily
Loan portfolio by credit quality indicator
Portfolio, total	101,976	121,599
Commercial real estate: multifamily | Pass
Loan portfolio by credit quality indicator
Portfolio, total	101,538	121,188
Commercial real estate: multifamily | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	56
Commercial real estate: multifamily | Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	382	411
Residential: first lien
Loan portfolio by credit quality indicator
Portfolio, total	462,503	495,722
Residential: first lien | Pass
Loan portfolio by credit quality indicator
Portfolio, total	452,856	490,924
Residential: first lien | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	366	132
Residential: first lien | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	33	974
Residential: first lien | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	6,496	1,402
Residential: first lien | Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	2,752	2,290
Residential: junior lien
Loan portfolio by credit quality indicator
Portfolio, total	379,809	400,148
Residential: junior lien | Pass
Loan portfolio by credit quality indicator
Portfolio, total	376,000	397,861
Residential: junior lien | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	79
Residential: junior lien | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	320	319
Residential: junior lien | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	1,500	6
Residential: junior lien | Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	1,910	1,962
Consumer
Loan portfolio by credit quality indicator
Portfolio, total	83,199	94,109
Consumer | Pass
Loan portfolio by credit quality indicator
Portfolio, total	82,083	92,714
Consumer | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	37	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	1,079	1,334
Foreign
Loan portfolio by credit quality indicator
Portfolio, total	209,050	230,005
Foreign | Pass
Loan portfolio by credit quality indicator
Portfolio, total	208,960	229,898
Foreign | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	45	61
Foreign | Watch List - Impaired
Loan portfolio by credit quality indicator
Portfolio, total	$ 45	$ 46