Allowance for Probable Loan Losses (Impairment By Loan Class) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Individually Evaluated for Impairment, Recorded Investment	$ 35,440	$ 63,204
Loans Individually Evaluated for Impairment, Allowance	576	434
Loans Collectively Evaluated for Impairment, Recorded Investment	6,403,980	6,284,968
Loans Collectively Evaluated for Impairment, Allowance	62,277	67,253
Commercial
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Individually Evaluated for Impairment, Recorded Investment	17,933	17,947
Loans Individually Evaluated for Impairment, Allowance	460	300
Loans Collectively Evaluated for Impairment, Recorded Investment	1,072,061	1,068,520
Loans Collectively Evaluated for Impairment, Allowance	16,136	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	2,166	2,455
Loans Individually Evaluated for Impairment, Allowance	116	116
Loans Collectively Evaluated for Impairment, Recorded Investment	1,779,787	1,681,095
Loans Collectively Evaluated for Impairment, Allowance	14,587	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	6,020	33,123
Loans Individually Evaluated for Impairment, Allowance		18
Loans Collectively Evaluated for Impairment, Recorded Investment	2,011,747	2,010,162
Loans Collectively Evaluated for Impairment, Allowance	18,296	16,645
Commercial Real Estate: Multifamily
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Individually Evaluated for Impairment, Recorded Investment	664	476
Loans Collectively Evaluated for Impairment, Recorded Investment	212,916	192,440
Loans Collectively Evaluated for Impairment, Allowance	1,574	1,109
Residential First Lien
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Individually Evaluated for Impairment, Recorded Investment	6,387	6,852
Loans Collectively Evaluated for Impairment, Recorded Investment	420,951	425,925
Loans Collectively Evaluated for Impairment, Allowance	3,591	2,950
Residential Junior Lien
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Individually Evaluated for Impairment, Recorded Investment	799	723
Loans Collectively Evaluated for Impairment, Recorded Investment	708,784	700,025
Loans Collectively Evaluated for Impairment, Allowance	6,793	6,103
Consumer
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Individually Evaluated for Impairment, Recorded Investment	1,141	1,281
Loans Collectively Evaluated for Impairment, Recorded Investment	46,606	48,262
Loans Collectively Evaluated for Impairment, Allowance	450	440
Foreign
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Individually Evaluated for Impairment, Recorded Investment	330	347
Loans Collectively Evaluated for Impairment, Recorded Investment	151,128	158,539
Loans Collectively Evaluated for Impairment, Allowance	$ 850	$ 842