Allowance for Probable Loan Losses (Impairment By Loan Class) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Individually Evaluated for Impairment, Recorded Investment	$ 38,169	$ 48,405
Loans Individually Evaluated for Impairment, Allowance	434	961
Loans Collectively Evaluated for Impairment, Recorded Investment	6,164,096	5,916,283
Loans Collectively Evaluated for Impairment, Allowance	67,418	63,700
Commercial
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Individually Evaluated for Impairment, Recorded Investment	20,193	22,412
Loans Individually Evaluated for Impairment, Allowance	300
Loans Collectively Evaluated for Impairment, Recorded Investment	1,008,849	887,255
Loans Collectively Evaluated for Impairment, Allowance	26,695	25,649
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	2,428	4,776
Loans Individually Evaluated for Impairment, Allowance	116	371
Loans Collectively Evaluated for Impairment, Recorded Investment	1,702,376	1,712,099
Loans Collectively Evaluated for Impairment, Allowance	11,769	13,518
Commercial Real Estate: Farmland & Commercial
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Individually Evaluated for Impairment, Recorded Investment	5,966	10,810
Loans Individually Evaluated for Impairment, Allowance	18	546
Loans Collectively Evaluated for Impairment, Recorded Investment	1,969,876	1,932,260
Loans Collectively Evaluated for Impairment, Allowance	18,166	16,185
Commercial Real Estate: Multifamily
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Individually Evaluated for Impairment, Recorded Investment	496	552
Loans Collectively Evaluated for Impairment, Recorded Investment	166,881	139,914
Loans Collectively Evaluated for Impairment, Allowance	997	806
Residential First Lien
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Individually Evaluated for Impairment, Recorded Investment	6,651	6,836
Loans Individually Evaluated for Impairment, Allowance		44
Loans Collectively Evaluated for Impairment, Recorded Investment	421,204	415,068
Loans Collectively Evaluated for Impairment, Allowance	3,115	2,411
Residential Junior Lien
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Individually Evaluated for Impairment, Recorded Investment	762	978
Loans Collectively Evaluated for Impairment, Recorded Investment	679,754	609,340
Loans Collectively Evaluated for Impairment, Allowance	5,235	3,716
Consumer
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Individually Evaluated for Impairment, Recorded Investment	1,318	1,295
Loans Collectively Evaluated for Impairment, Recorded Investment	52,641	53,873
Loans Collectively Evaluated for Impairment, Allowance	513	531
Foreign
Loan loss allowances, impaired financing receivable, evaluated individually or collectively
Loans Individually Evaluated for Impairment, Recorded Investment	355	746
Loans Collectively Evaluated for Impairment, Recorded Investment	162,515	166,474
Loans Collectively Evaluated for Impairment, Allowance	$ 928	$ 884