Allowance for Probable Loan Losses (Impaired Loans) (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Interest Recognized
Total trouble debt restructuring	$ 16,301		$ 15,346
Period of time for determining the historical charge-off experience	3 years
Period of charge off for past due unsecured commercial loans	90 days
Class of Financing Receivable
Recorded investment:
Total impaired loans with related allowance	$ 16,952		1,894
Total impaired loans with no related allowance	49,881		61,310
Unpaid principal balance:
Total impaired loans with an allowance recorded	17,421		2,336
Total impaired loans with no related allowance recorded	78,388		89,664
Related Allowance	8,910		434
Average recorded investment:
Total impaired loans with an allowance recorded	16,964	$ 4,346	1,985
Total impaired loans with no related allowance recorded	50,280	40,286	72,417
Interest Recognized
Total impaired loans with no related allowance recorded	93	124	391
Commercial
Interest Recognized
Total trouble debt restructuring	6,742		6,910
Commercial | Class of Financing Receivable
Recorded investment:
Total impaired loans with related allowance	1,708
Total impaired loans with no related allowance	16,675		16,646
Unpaid principal balance:
Total impaired loans with an allowance recorded	1,992
Total impaired loans with no related allowance recorded	44,200		44,095
Related Allowance	471
Average recorded investment:
Total impaired loans with an allowance recorded	1,717	1,165
Total impaired loans with no related allowance recorded	16,820	20,789	19,615
Interest Recognized
Total impaired loans with no related allowance recorded	1	1	3
Commercial Real Estate:Other Construction and Land Development | Class of Financing Receivable
Recorded investment:
Total impaired loans with related allowance	144		1,300
Total impaired loans with no related allowance	2,271		2,310
Unpaid principal balance:
Total impaired loans with an allowance recorded	169		1,577
Total impaired loans with no related allowance recorded	2,451		2,455
Related Allowance	116		300
Average recorded investment:
Total impaired loans with an allowance recorded	144	1,299	1,346
Total impaired loans with no related allowance recorded	2,284	2,683	3,493
Commercial Real Estate: Farmland and Commercial
Interest Recognized
Total trouble debt restructuring	1,389
Commercial Real Estate: Farmland and Commercial | Class of Financing Receivable
Recorded investment:
Total impaired loans with related allowance	15,100		145
Total impaired loans with no related allowance	21,471		32,675
Unpaid principal balance:
Total impaired loans with an allowance recorded	15,260		169
Total impaired loans with no related allowance recorded	22,107		33,275
Related Allowance	8,323		116
Average recorded investment:
Total impaired loans with an allowance recorded	15,103	1,820	150
Total impaired loans with no related allowance recorded	21,488	6,751	38,536
Interest Recognized
Total impaired loans with no related allowance recorded		28
Commercial Real Estate: Multifamily | Class of Financing Receivable
Recorded investment:
Total impaired loans with related allowance			449
Total impaired loans with no related allowance	470		476
Unpaid principal balance:
Total impaired loans with an allowance recorded			590
Total impaired loans with no related allowance recorded	501		505
Related Allowance			18
Average recorded investment:
Total impaired loans with an allowance recorded			489
Total impaired loans with no related allowance recorded	471	539	511
Residential First Lien
Interest Recognized
Total trouble debt restructuring	5,974		6,140
Residential First Lien | Class of Financing Receivable
Recorded investment:
Total impaired loans with no related allowance	6,747		6,852
Unpaid principal balance:
Total impaired loans with no related allowance recorded	6,868		6,968
Average recorded investment:
Total impaired loans with an allowance recorded		62
Total impaired loans with no related allowance recorded	6,960	6,558	7,249
Interest Recognized
Total impaired loans with no related allowance recorded	77	79	324
Residential Junior Lien
Interest Recognized
Total trouble debt restructuring	701		712
Residential Junior Lien | Class of Financing Receivable
Recorded investment:
Total impaired loans with no related allowance	704		723
Unpaid principal balance:
Total impaired loans with no related allowance recorded	715		736
Average recorded investment:
Total impaired loans with no related allowance recorded	708	972	970
Interest Recognized
Total impaired loans with no related allowance recorded	10	12	45
Consumer
Interest Recognized
Total trouble debt restructuring	1,157		1,237
Consumer | Class of Financing Receivable
Recorded investment:
Total impaired loans with no related allowance	1,205		1,281
Unpaid principal balance:
Total impaired loans with no related allowance recorded	1,208		1,283
Average recorded investment:
Total impaired loans with no related allowance recorded	1,208	1,246	1,293
Interest Recognized
Total impaired loans with no related allowance recorded	1		3
Foreign
Interest Recognized
Total trouble debt restructuring	338		347
Foreign | Class of Financing Receivable
Recorded investment:
Total impaired loans with no related allowance	338		347
Unpaid principal balance:
Total impaired loans with no related allowance recorded	338		347
Average recorded investment:
Total impaired loans with no related allowance recorded	341	748	750
Interest Recognized
Total impaired loans with no related allowance recorded	$ 4	$ 4	$ 16