Allowance for Probable Loan Losses (Aging By Loan Class) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financing receivable recorded investment
Past due	$ 84,229	$ 77,053
Past due 90 days or Greater and Still Accruing	7,449	7,257
Loans, current	6,390,912	6,271,119
Total loans	6,475,141	6,348,172
30 to 59 Days
Financing receivable recorded investment
Past due	51,390	15,855
60 to 89 Days
Financing receivable recorded investment
Past due	4,392	4,429
90 Days or Greater
Financing receivable recorded investment
Past due	28,447	56,769
Commercial
Financing receivable recorded investment
Past due	33,259	22,496
Past due 90 days or Greater and Still Accruing	886	537
Loans, current	1,045,306	1,063,971
Total loans	1,078,565	1,086,467
Commercial | 30 to 59 Days
Financing receivable recorded investment
Past due	14,468	3,790
Commercial | 60 to 89 Days
Financing receivable recorded investment
Past due	1,381	398
Commercial | 90 Days or Greater
Financing receivable recorded investment
Past due	17,410	18,308
Commercial Real Estate:Other Construction and Land Development
Financing receivable recorded investment
Past due	3,682	1,482
Past due 90 days or Greater and Still Accruing	100	6
Loans, current	1,874,758	1,682,068
Total loans	1,878,440	1,683,550
Commercial Real Estate:Other Construction and Land Development | 30 to 59 Days
Financing receivable recorded investment
Past due	2,596	354
Commercial Real Estate:Other Construction and Land Development | 60 to 89 Days
Financing receivable recorded investment
Past due	81	308
Commercial Real Estate:Other Construction and Land Development | 90 Days or Greater
Financing receivable recorded investment
Past due	1,005	820
Commercial Real Estate: Farmland and Commercial
Financing receivable recorded investment
Past due	32,009	35,576
Past due 90 days or Greater and Still Accruing	789	954
Loans, current	1,949,545	2,007,709
Total loans	1,981,554	2,043,285
Commercial Real Estate: Farmland and Commercial | 30 to 59 Days
Financing receivable recorded investment
Past due	27,621	3,925
Commercial Real Estate: Farmland and Commercial | 60 to 89 Days
Financing receivable recorded investment
Past due	733	518
Commercial Real Estate: Farmland and Commercial | 90 Days or Greater
Financing receivable recorded investment
Past due	3,655	31,133
Commercial Real Estate: Multifamily
Financing receivable recorded investment
Past due	868	560
Loans, current	186,477	192,356
Total loans	187,345	192,916
Commercial Real Estate: Multifamily | 30 to 59 Days
Financing receivable recorded investment
Past due	282	84
Commercial Real Estate: Multifamily | 90 Days or Greater
Financing receivable recorded investment
Past due	586	476
Residential First Lien
Financing receivable recorded investment
Past due	8,276	10,848
Past due 90 days or Greater and Still Accruing	3,435	3,861
Loans, current	418,270	421,929
Total loans	426,546	432,777
Residential First Lien | 30 to 59 Days
Financing receivable recorded investment
Past due	4,029	4,295
Residential First Lien | 60 to 89 Days
Financing receivable recorded investment
Past due	709	2,458
Residential First Lien | 90 Days or Greater
Financing receivable recorded investment
Past due	3,538	4,095
Residential Junior Lien
Financing receivable recorded investment
Past due	2,153	3,000
Past due 90 days or Greater and Still Accruing	1,120	1,099
Loans, current	718,933	697,748
Total loans	721,086	700,748
Residential Junior Lien | 30 to 59 Days
Financing receivable recorded investment
Past due	746	1,310
Residential Junior Lien | 60 to 89 Days
Financing receivable recorded investment
Past due	287	580
Residential Junior Lien | 90 Days or Greater
Financing receivable recorded investment
Past due	1,120	1,110
Consumer
Financing receivable recorded investment
Past due	526	1,126
Past due 90 days or Greater and Still Accruing	38	133
Loans, current	46,120	48,417
Total loans	46,646	49,543
Consumer | 30 to 59 Days
Financing receivable recorded investment
Past due	410	868
Consumer | 60 to 89 Days
Financing receivable recorded investment
Past due	64	98
Consumer | 90 Days or Greater
Financing receivable recorded investment
Past due	52	160
Foreign
Financing receivable recorded investment
Past due	3,456	1,965
Past due 90 days or Greater and Still Accruing	1,081	667
Loans, current	151,503	156,921
Total loans	154,959	158,886
Foreign | 30 to 59 Days
Financing receivable recorded investment
Past due	1,238	1,229
Foreign | 60 to 89 Days
Financing receivable recorded investment
Past due	1,137	69
Foreign | 90 Days or Greater
Financing receivable recorded investment
Past due	$ 1,081	$ 667