Allowance for Probable Loan Losses (Impairment By Loan Class) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Individually Evaluated for Impairment, Recorded Investment	$ 66,833	$ 63,204
Loans Individually Evaluated for Impairment, Allowance	8,910	434
Loans Collectively Evaluated for Impairment, Recorded Investment	6,386,270	6,284,968
Loans Collectively Evaluated for Impairment, Allowance	58,244	67,253
Commercial
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Individually Evaluated for Impairment, Recorded Investment	18,384	17,947
Loans Individually Evaluated for Impairment, Allowance	471	300
Loans Collectively Evaluated for Impairment, Recorded Investment	1,104,712	1,068,520
Loans Collectively Evaluated for Impairment, Allowance	20,220	27,605
Commercial Real Estate:Other Construction and Land Development
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Individually Evaluated for Impairment, Recorded Investment	2,415	2,455
Loans Individually Evaluated for Impairment, Allowance	116	116
Loans Collectively Evaluated for Impairment, Recorded Investment	1,723,214	1,681,095
Loans Collectively Evaluated for Impairment, Allowance	11,682	11,559
Commercial Real Estate: Farmland and Commercial
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Individually Evaluated for Impairment, Recorded Investment	36,570	33,123
Loans Individually Evaluated for Impairment, Allowance	8,323	18
Loans Collectively Evaluated for Impairment, Recorded Investment	2,055,269	2,010,162
Loans Collectively Evaluated for Impairment, Allowance	16,480	16,645
Commercial Real Estate: Multifamily
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Individually Evaluated for Impairment, Recorded Investment	470	476
Loans Collectively Evaluated for Impairment, Recorded Investment	178,846	192,440
Loans Collectively Evaluated for Impairment, Allowance	947	1,109
Residential First Lien
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Individually Evaluated for Impairment, Recorded Investment	6,747	6,852
Loans Collectively Evaluated for Impairment, Recorded Investment	420,932	425,925
Loans Collectively Evaluated for Impairment, Allowance	3,006	2,950
Residential Junior Lien
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Individually Evaluated for Impairment, Recorded Investment	704	723
Loans Collectively Evaluated for Impairment, Recorded Investment	700,551	700,025
Loans Collectively Evaluated for Impairment, Allowance	4,689	6,103
Consumer
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Individually Evaluated for Impairment, Recorded Investment	1,205	1,281
Loans Collectively Evaluated for Impairment, Recorded Investment	45,573	48,262
Loans Collectively Evaluated for Impairment, Allowance	437	440
Foreign
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Individually Evaluated for Impairment, Recorded Investment	338	347
Loans Collectively Evaluated for Impairment, Recorded Investment	157,173	158,539
Loans Collectively Evaluated for Impairment, Allowance	$ 783	$ 842