Allowance for Probable Loan Losses (Aging By Loan Class) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Financing receivable recorded investment
Past due	$ 83,538	$ 61,400
Past due 90 days or Greater and Still Accruing	7,280	5,226
Loans, current	6,118,726	5,903,288
Total loans	6,202,264	5,964,688
30 to 59 Days
Financing receivable recorded investment
Past due	20,025	17,299
60 to 89 Days
Financing receivable recorded investment
Past due	31,695	6,327
90 Days or Greater
Financing receivable recorded investment
Past due	31,818	37,774
Commercial
Financing receivable recorded investment
Past due	24,204	26,033
Past due 90 days or Greater and Still Accruing	706	392
Loans, current	1,004,837	883,634
Total loans	1,029,041	909,667
Commercial | 30 to 59 Days
Financing receivable recorded investment
Past due	3,604	4,081
Commercial | 60 to 89 Days
Financing receivable recorded investment
Past due	352	829
Commercial | 90 Days or Greater
Financing receivable recorded investment
Past due	20,248	21,123
Commercial Real Estate: Other Construction and Land Development
Financing receivable recorded investment
Past due	1,308	6,354
Past due 90 days or Greater and Still Accruing	49	9
Loans, current	1,703,496	1,710,521
Total loans	1,704,804	1,716,875
Commercial Real Estate: Other Construction and Land Development | 30 to 59 Days
Financing receivable recorded investment
Past due	444	1,502
Commercial Real Estate: Other Construction and Land Development | 60 to 89 Days
Financing receivable recorded investment
Past due		396
Commercial Real Estate: Other Construction and Land Development | 90 Days or Greater
Financing receivable recorded investment
Past due	864	4,456
Commercial Real Estate: Farmland & Commercial
Financing receivable recorded investment
Past due	42,747	12,658
Past due 90 days or Greater and Still Accruing	676	289
Loans, current	1,933,095	1,930,412
Total loans	1,975,842	1,943,070
Commercial Real Estate: Farmland & Commercial | 30 to 59 Days
Financing receivable recorded investment
Past due	9,117	3,454
Commercial Real Estate: Farmland & Commercial | 60 to 89 Days
Financing receivable recorded investment
Past due	29,568	3,054
Commercial Real Estate: Farmland & Commercial | 90 Days or Greater
Financing receivable recorded investment
Past due	4,062	6,150
Commercial Real Estate: Multifamily
Financing receivable recorded investment
Past due	645	596
Loans, current	166,732	139,870
Total loans	167,377	140,466
Commercial Real Estate: Multifamily | 30 to 59 Days
Financing receivable recorded investment
Past due	149	44
Commercial Real Estate: Multifamily | 90 Days or Greater
Financing receivable recorded investment
Past due	496	552
Residential First Lien
Financing receivable recorded investment
Past due	8,776	11,108
Past due 90 days or Greater and Still Accruing	4,382	3,756
Loans, current	419,079	410,796
Total loans	427,855	421,904
Residential First Lien | 30 to 59 Days
Financing receivable recorded investment
Past due	3,161	5,615
Residential First Lien | 60 to 89 Days
Financing receivable recorded investment
Past due	964	1,350
Residential First Lien | 90 Days or Greater
Financing receivable recorded investment
Past due	4,651	4,143
Residential Junior Lien
Financing receivable recorded investment
Past due	2,471	1,480
Past due 90 days or Greater and Still Accruing	682	382
Loans, current	678,045	608,838
Total loans	680,516	610,318
Residential Junior Lien | 30 to 59 Days
Financing receivable recorded investment
Past due	1,369	762
Residential Junior Lien | 60 to 89 Days
Financing receivable recorded investment
Past due	420	178
Residential Junior Lien | 90 Days or Greater
Financing receivable recorded investment
Past due	682	540
Consumer
Financing receivable recorded investment
Past due	1,390	1,418
Past due 90 days or Greater and Still Accruing	523	387
Loans, current	52,569	53,750
Total loans	53,959	55,168
Consumer | 30 to 59 Days
Financing receivable recorded investment
Past due	713	910
Consumer | 60 to 89 Days
Financing receivable recorded investment
Past due	124	95
Consumer | 90 Days or Greater
Financing receivable recorded investment
Past due	553	413
Foreign
Financing receivable recorded investment
Past due	1,997	1,753
Past due 90 days or Greater and Still Accruing	262	11
Loans, current	160,873	165,467
Total loans	162,870	167,220
Foreign | 30 to 59 Days
Financing receivable recorded investment
Past due	1,468	931
Foreign | 60 to 89 Days
Financing receivable recorded investment
Past due	267	425
Foreign | 90 Days or Greater
Financing receivable recorded investment
Past due	$ 262	$ 397