Allowance for Probable Loan Losses (Aging By Loan Class) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Financing receivable recorded investment
Past due 90 days or Greater and Still Accruing	$ 7,626	$ 40,674
Past due	116,561	75,833
Loans, current	6,778,581	6,485,456
Total loans	6,895,142	6,561,289
30 to 59 Days
Financing receivable recorded investment
Past due	22,870	15,978
60 to 89 Days
Financing receivable recorded investment
Past due	77,913	6,011
90 Days or Greater
Financing receivable recorded investment
Past due	15,778	53,844
Commercial
Financing receivable recorded investment
Past due 90 days or Greater and Still Accruing	1,477	10,890
Past due	11,803	25,591
Loans, current	1,283,933	1,103,378
Total loans	1,295,736	1,128,969
Commercial | 30 to 59 Days
Financing receivable recorded investment
Past due	3,493	4,651
Commercial | 60 to 89 Days
Financing receivable recorded investment
Past due	902	1,089
Commercial | 90 Days or Greater
Financing receivable recorded investment
Past due	7,408	19,851
Commercial Real Estate:Other Construction and Land Development
Financing receivable recorded investment
Past due 90 days or Greater and Still Accruing	286	16
Past due	28,581	3,356
Loans, current	2,015,829	1,882,875
Total loans	2,044,410	1,886,231
Commercial Real Estate:Other Construction and Land Development | 30 to 59 Days
Financing receivable recorded investment
Past due	8,977	727
Commercial Real Estate:Other Construction and Land Development | 60 to 89 Days
Financing receivable recorded investment
Past due	18,504	1,707
Commercial Real Estate:Other Construction and Land Development | 90 Days or Greater
Financing receivable recorded investment
Past due	1,100	922
Commercial Real Estate: Farmland and Commercial
Financing receivable recorded investment
Past due 90 days or Greater and Still Accruing	737	24,910
Past due	60,605	30,951
Loans, current	1,906,506	1,918,947
Total loans	1,967,111	1,949,898
Commercial Real Estate: Farmland and Commercial | 30 to 59 Days
Financing receivable recorded investment
Past due	3,210	2,928
Commercial Real Estate: Farmland and Commercial | 60 to 89 Days
Financing receivable recorded investment
Past due	56,125	784
Commercial Real Estate: Farmland and Commercial | 90 Days or Greater
Financing receivable recorded investment
Past due	1,270	27,239
Commercial Real Estate: Multifamily
Financing receivable recorded investment
Past due 90 days or Greater and Still Accruing	764	71
Past due	1,579	1,505
Loans, current	240,510	224,752
Total loans	242,089	226,257
Commercial Real Estate: Multifamily | 30 to 59 Days
Financing receivable recorded investment
Past due	24	927
Commercial Real Estate: Multifamily | 60 to 89 Days
Financing receivable recorded investment
Past due	290
Commercial Real Estate: Multifamily | 90 Days or Greater
Financing receivable recorded investment
Past due	1,265	578
Residential First Lien
Financing receivable recorded investment
Past due 90 days or Greater and Still Accruing	2,930	3,079
Past due	7,433	9,037
Loans, current	431,039	436,763
Total loans	438,472	445,800
Residential First Lien | 30 to 59 Days
Financing receivable recorded investment
Past due	2,961	3,998
Residential First Lien | 60 to 89 Days
Financing receivable recorded investment
Past due	1,169	1,677
Residential First Lien | 90 Days or Greater
Financing receivable recorded investment
Past due	3,303	3,362
Residential Junior Lien
Financing receivable recorded investment
Past due 90 days or Greater and Still Accruing	1,216	937
Past due	2,837	2,881
Loans, current	710,455	724,420
Total loans	713,292	727,301
Residential Junior Lien | 30 to 59 Days
Financing receivable recorded investment
Past due	1,316	1,155
Residential Junior Lien | 60 to 89 Days
Financing receivable recorded investment
Past due	305	618
Residential Junior Lien | 90 Days or Greater
Financing receivable recorded investment
Past due	1,216	1,108
Consumer
Financing receivable recorded investment
Past due 90 days or Greater and Still Accruing	69	32
Past due	967	550
Loans, current	46,105	45,766
Total loans	47,072	46,316
Consumer | 30 to 59 Days
Financing receivable recorded investment
Past due	770	486
Consumer | 60 to 89 Days
Financing receivable recorded investment
Past due	128	19
Consumer | 90 Days or Greater
Financing receivable recorded investment
Past due	69	45
Foreign
Financing receivable recorded investment
Past due 90 days or Greater and Still Accruing	147	739
Past due	2,756	1,962
Loans, current	144,204	148,555
Total loans	146,960	150,517
Foreign | 30 to 59 Days
Financing receivable recorded investment
Past due	2,119	1,106
Foreign | 60 to 89 Days
Financing receivable recorded investment
Past due	490	117
Foreign | 90 Days or Greater
Financing receivable recorded investment
Past due	$ 147	$ 739