Allowance for Probable Loan Losses (Portfolio Credit Quality By Loan Class) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Loan portfolio by credit quality indicator
Portfolio, total	$ 6,475,141	$ 6,348,172
Pass
Loan portfolio by credit quality indicator
Portfolio, total	6,055,035	5,911,001
Special Review
Loan portfolio by credit quality indicator
Portfolio, total	61,544	6,612
Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	85,273	45,662
Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	240,797	321,693
Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	32,492	63,204
Commercial
Loan portfolio by credit quality indicator
Portfolio, total	1,078,565	1,086,467
Commercial | Pass
Loan portfolio by credit quality indicator
Portfolio, total	959,115	905,707
Commercial | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	449
Commercial | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	38,469	3,170
Commercial | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	63,296	159,643
Commercial | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	17,236	17,947
Commercial Real Estate:Other Construction and Land Development
Loan portfolio by credit quality indicator
Portfolio, total	1,878,440	1,683,550
Commercial Real Estate:Other Construction and Land Development | Pass
Loan portfolio by credit quality indicator
Portfolio, total	1,811,960	1,616,604
Commercial Real Estate:Other Construction and Land Development | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	1,662	1,288
Commercial Real Estate:Other Construction and Land Development | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	7,171	672
Commercial Real Estate:Other Construction and Land Development | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	55,517	62,531
Commercial Real Estate:Other Construction and Land Development | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	2,130	2,455
Commercial Real Estate: Farmland and Commercial
Loan portfolio by credit quality indicator
Portfolio, total	1,981,554	2,043,285
Commercial Real Estate: Farmland and Commercial | Pass
Loan portfolio by credit quality indicator
Portfolio, total	1,759,210	1,863,763
Commercial Real Estate: Farmland and Commercial | Special Review
Loan portfolio by credit quality indicator
Portfolio, total	59,433	5,134
Commercial Real Estate: Farmland and Commercial | Watch List - Pass
Loan portfolio by credit quality indicator
Portfolio, total	38,622	41,820
Commercial Real Estate: Farmland and Commercial | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	120,132	99,445
Commercial Real Estate: Farmland and Commercial | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	4,157	33,123
Commercial Real Estate: Multifamily
Loan portfolio by credit quality indicator
Portfolio, total	187,345	192,916
Commercial Real Estate: Multifamily | Pass
Loan portfolio by credit quality indicator
Portfolio, total	185,823	192,440
Commercial Real Estate: Multifamily | Watch List - Substandard
Loan portfolio by credit quality indicator
Portfolio, total	936
Commercial Real Estate: Multifamily | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	586	476
Residential First Lien
Loan portfolio by credit quality indicator
Portfolio, total	426,546	432,777
Residential First Lien | Pass
Loan portfolio by credit quality indicator
Portfolio, total	419,328	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	916	74
Residential First Lien | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	6,173	6,852
Residential Junior Lien
Loan portfolio by credit quality indicator
Portfolio, total	721,086	700,748
Residential Junior Lien | Pass
Loan portfolio by credit quality indicator
Portfolio, total	719,464	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	882
Residential Junior Lien | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	740	723
Consumer
Loan portfolio by credit quality indicator
Portfolio, total	46,646	49,543
Consumer | Pass
Loan portfolio by credit quality indicator
Portfolio, total	45,482	48,262
Consumer | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	1,164	1,281
Foreign
Loan portfolio by credit quality indicator
Portfolio, total	154,959	158,886
Foreign | Pass
Loan portfolio by credit quality indicator
Portfolio, total	154,653	158,539
Foreign | Watch - List Impaired
Loan portfolio by credit quality indicator
Portfolio, total	$ 306	$ 347