Allowance for Probable Loan Losses (Portfolio Credit Quality By Loan Class) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Loan portfolio by credit quality indicator
Portfolio, total	$ 6,439,420	$ 6,348,172
Pass
Loan portfolio by credit quality indicator
Portfolio, total	6,021,949	5,911,001
Special Review
Loan portfolio by credit quality indicator
Portfolio, total	58,494	6,612
Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	48,585	45,662
Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	274,952	321,693
Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	35,440	63,204
Commercial
Loan portfolio by credit quality indicator
Portfolio, total	1,089,994	1,086,467
Commercial | Pass
Loan portfolio by credit quality indicator
Portfolio, total	951,055	905,707
Commercial | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	256
Commercial | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	1,034	3,170
Commercial | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	119,716	159,643
Commercial | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	17,933	17,947
Commercial Real Estate:Other Construction and Land Development
Loan portfolio by credit quality indicator
Portfolio, total	1,781,953	1,683,550
Commercial Real Estate:Other Construction and Land Development | Pass
Loan portfolio by credit quality indicator
Portfolio, total	1,715,667	1,616,604
Commercial Real Estate:Other Construction and Land Development | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	1,115	1,288
Commercial Real Estate:Other Construction and Land Development | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	7,169	672
Commercial Real Estate:Other Construction and Land Development | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	55,836	62,531
Commercial Real Estate:Other Construction and Land Development | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	2,166	2,455
Commercial Real Estate: Farmland and Commercial
Loan portfolio by credit quality indicator
Portfolio, total	2,017,767	2,043,285
Commercial Real Estate: Farmland and Commercial | Pass
Loan portfolio by credit quality indicator
Portfolio, total	1,817,420	1,863,763
Commercial Real Estate: Farmland and Commercial | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	57,123	5,134
Commercial Real Estate: Farmland and Commercial | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	39,372	41,820
Commercial Real Estate: Farmland and Commercial | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	97,832	99,445
Commercial Real Estate: Farmland and Commercial | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	6,020	33,123
Commercial Real Estate: Multifamily
Loan portfolio by credit quality indicator
Portfolio, total	213,580	192,916
Commercial Real Estate: Multifamily | Pass
Loan portfolio by credit quality indicator
Portfolio, total	211,972	192,440
Commercial Real Estate: Multifamily | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	944
Commercial Real Estate: Multifamily | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	664	476
Residential First Lien
Loan portfolio by credit quality indicator
Portfolio, total	427,338	432,777
Residential First Lien | Pass
Loan portfolio by credit quality indicator
Portfolio, total	420,198	425,811
Residential First Lien | Special Review
Loan portfolio by credit quality indicator
Portfolio, total		40
Residential First Lien | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	129
Residential First Lien | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	624	74
Residential First Lien | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	6,387	6,852
Residential Junior Lien
Loan portfolio by credit quality indicator
Portfolio, total	709,583	700,748
Residential Junior Lien | Pass
Loan portfolio by credit quality indicator
Portfolio, total	707,903	699,875
Residential Junior Lien | Special Review
Loan portfolio by credit quality indicator
Portfolio, total		150
Residential Junior Lien | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	881
Residential Junior Lien | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	799	723
Consumer
Loan portfolio by credit quality indicator
Portfolio, total	47,747	49,543
Consumer | Pass
Loan portfolio by credit quality indicator
Portfolio, total	46,606	48,262
Consumer | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	1,141	1,281
Foreign
Loan portfolio by credit quality indicator
Portfolio, total	151,458	158,886
Foreign | Pass
Loan portfolio by credit quality indicator
Portfolio, total	151,128	158,539
Foreign | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	$ 330	$ 347