Allowance for Probable Loan Losses (Portfolio Credit Quality By Loan Class) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Loan portfolio by credit quality indicator
Portfolio, total	$ 6,453,103	$ 6,348,172
Pass
Loan portfolio by credit quality indicator
Portfolio, total	5,981,361	5,911,001
Special Review
Loan portfolio by credit quality indicator
Portfolio, total	65,967	6,612
Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	48,067	45,662
Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	290,875	321,693
Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	66,833	63,204
Commercial
Loan portfolio by credit quality indicator
Portfolio, total	1,123,096	1,086,467
Commercial | Pass
Loan portfolio by credit quality indicator
Portfolio, total	970,585	905,707
Commercial | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	1,299	3,170
Commercial | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	132,828	159,643
Commercial | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	18,384	17,947
Commercial Real Estate:Other Construction and Land Development
Loan portfolio by credit quality indicator
Portfolio, total	1,725,629	1,683,550
Commercial Real Estate:Other Construction and Land Development | Pass
Loan portfolio by credit quality indicator
Portfolio, total	1,658,783	1,616,604
Commercial Real Estate:Other Construction and Land Development | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	3,532	1,288
Commercial Real Estate:Other Construction and Land Development | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	667	672
Commercial Real Estate:Other Construction and Land Development | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	60,232	62,531
Commercial Real Estate:Other Construction and Land Development | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	2,415	2,455
Commercial Real Estate: Farmland and Commercial
Loan portfolio by credit quality indicator
Portfolio, total	2,091,839	2,043,285
Commercial Real Estate: Farmland and Commercial | Pass
Loan portfolio by credit quality indicator
Portfolio, total	1,851,726	1,863,763
Commercial Real Estate: Farmland and Commercial | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	61,413	5,134
Commercial Real Estate: Farmland and Commercial | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	45,022	41,820
Commercial Real Estate: Farmland and Commercial | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	97,108	99,445
Commercial Real Estate: Farmland and Commercial | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	36,570	33,123
Commercial Real Estate: Multifamily
Loan portfolio by credit quality indicator
Portfolio, total	179,316	192,916
Commercial Real Estate: Multifamily | Pass
Loan portfolio by credit quality indicator
Portfolio, total	177,894	192,440
Commercial Real Estate: Multifamily | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	952
Commercial Real Estate: Multifamily | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	470	476
Residential First Lien
Loan portfolio by credit quality indicator
Portfolio, total	427,679	432,777
Residential First Lien | Pass
Loan portfolio by credit quality indicator
Portfolio, total	420,060	425,811
Residential First Lien | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	38	40
Residential First Lien | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	127
Residential First Lien | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	707	74
Residential First Lien | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	6,747	6,852
Residential Junior Lien
Loan portfolio by credit quality indicator
Portfolio, total	701,255	700,748
Residential Junior Lien | Pass
Loan portfolio by credit quality indicator
Portfolio, total	699,567	699,875
Residential Junior Lien | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	984	150
Residential Junior Lien | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	704	723
Consumer
Loan portfolio by credit quality indicator
Portfolio, total	46,778	49,543
Consumer | Pass
Loan portfolio by credit quality indicator
Portfolio, total	45,573	48,262
Consumer | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	1,205	1,281
Foreign
Loan portfolio by credit quality indicator
Portfolio, total	157,511	158,886
Foreign | Pass
Loan portfolio by credit quality indicator
Portfolio, total	157,173	158,539
Foreign | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	$ 338	$ 347