Allowance for Probable Loan Losses (Non-accrual Basis By Loan Class) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Loan loss allowances, financing receivable past due
Non-accrual loans, total	$ 58,626	$ 54,730
Commercial
Loan loss allowances, financing receivable past due
Non-accrual loans, total	18,347	17,909
Commercial Real Estate:Other Construction and Land Development
Loan loss allowances, financing receivable past due
Non-accrual loans, total	2,415	2,455
Commercial Real Estate: Farmland and Commercial
Loan loss allowances, financing receivable past due
Non-accrual loans, total	36,570	33,123
Commercial Real Estate: Multifamily
Loan loss allowances, financing receivable past due
Non-accrual loans, total	470	476
Residential First Lien
Loan loss allowances, financing receivable past due
Non-accrual loans, total	773	712
Residential Junior Lien
Loan loss allowances, financing receivable past due
Non-accrual loans, total	3	11
Consumer
Loan loss allowances, financing receivable past due
Non-accrual loans, total	$ 48	$ 44