Allowance for Probable Loan Losses (Non-accrual Basis By Loan Class) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Loan loss allowances, financing receivable past due
Non-accrual loans, total	$ 24,300	$ 54,730
Commercial
Loan loss allowances, financing receivable past due
Non-accrual loans, total	17,199	17,909
Commercial Real Estate:Other Construction and Land Development
Loan loss allowances, financing receivable past due
Non-accrual loans, total	2,130	2,455
Commercial Real Estate: Farmland and Commercial
Loan loss allowances, financing receivable past due
Non-accrual loans, total	4,157	33,123
Commercial Real Estate: Multifamily
Loan loss allowances, financing receivable past due
Non-accrual loans, total	586	476
Residential First Lien
Loan loss allowances, financing receivable past due
Non-accrual loans, total	197	712
Residential Junior Lien
Loan loss allowances, financing receivable past due
Non-accrual loans, total		11
Consumer
Loan loss allowances, financing receivable past due
Non-accrual loans, total	$ 31	$ 44