Allowance for Probable Loan Losses (Impaired Loans) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Interest Recognized
Total trouble debt restructuring	$ 18,220		$ 18,220		$ 22,418
Period of charge off for past due unsecured commercial loans			90 days
Class of Financing Receivable
Recorded investment:
Total impaired loans with related allowance	2,408		$ 2,408		4,828
Total impaired loans with no related allowance	35,761		35,761		42,577
Unpaid principal balance:
Total impaired loans with an allowance recorded	4,624		4,624		5,981
Total impaired loans with no related allowance recorded	64,241		64,241		74,677
Related Allowance	434		434		961
Average recorded investment:
Total impaired loans with an allowance recorded	2,864	$ 7,442	3,086	$ 9,040	5,821
Total impaired loans with no related allowance recorded	38,855	33,532	42,273	36,577	40,450
Interest Recognized
Total impaired loans with an allowance recorded		24		73
Total impaired loans with no related allowance recorded	120	109	371	338	547
Commercial
Interest Recognized
Total trouble debt restructuring	9,072		9,072		10,710
Commercial | Class of Financing Receivable
Recorded investment:
Total impaired loans with related allowance	1,760		1,760
Total impaired loans with no related allowance	18,433		18,433		21,412
Unpaid principal balance:
Total impaired loans with an allowance recorded	3,865		3,865
Total impaired loans with no related allowance recorded	45,878		45,878		50,737
Related Allowance	300		300
Average recorded investment:
Total impaired loans with an allowance recorded	2,215	2,247	2,439	3,239
Total impaired loans with no related allowance recorded	18,547	13,813	18,931	15,234	19,354
Interest Recognized
Total impaired loans with no related allowance recorded	1	1	2	3	3
Commercial Real Estate: Other Construction and Land Development | Class of Financing Receivable
Recorded investment:
Total impaired loans with related allowance	148		148		1,958
Total impaired loans with no related allowance	2,280		2,280		2,818
Unpaid principal balance:
Total impaired loans with an allowance recorded	169		169		1,971
Total impaired loans with no related allowance recorded	2,397		2,397		4,419
Related Allowance	116		116		371
Average recorded investment:
Total impaired loans with an allowance recorded	149	159	151	162	2,512
Total impaired loans with no related allowance recorded	2,296	3,924	3,456	4,927	2,336
Interest Recognized
Total impaired loans with no related allowance recorded		14		53	67
Commercial Real Estate: Farmland & Commercial
Interest Recognized
Total trouble debt restructuring	590		590		3,086
Commercial Real Estate: Farmland & Commercial | Class of Financing Receivable
Recorded investment:
Total impaired loans with related allowance	500		500		2,808
Total impaired loans with no related allowance	5,466		5,466		8,002
Unpaid principal balance:
Total impaired loans with an allowance recorded	590		590		3,948
Total impaired loans with no related allowance recorded	6,235		6,235		9,054
Related Allowance	18		18		546
Average recorded investment:
Total impaired loans with an allowance recorded	500	5,036	496	5,639	3,247
Total impaired loans with no related allowance recorded	7,936	5,513	9,456	6,469	8,523
Interest Recognized
Total impaired loans with an allowance recorded		24		73
Total impaired loans with no related allowance recorded	21	3	79	10	110
Commercial Real Estate: Multifamily | Class of Financing Receivable
Recorded investment:
Total impaired loans with related allowance					62
Total impaired loans with no related allowance	496		496		552
Unpaid principal balance:
Total impaired loans with an allowance recorded					62
Total impaired loans with no related allowance recorded	518		518		562
Related Allowance					44
Average recorded investment:
Total impaired loans with an allowance recorded					62
Total impaired loans with no related allowance recorded	502	577	521	349	401
Residential First Lien
Interest Recognized
Total trouble debt restructuring	6,187		6,187		6,181
Residential First Lien | Class of Financing Receivable
Recorded investment:
Total impaired loans with no related allowance	6,651		6,651		6,774
Unpaid principal balance:
Total impaired loans with no related allowance recorded	6,765		6,765		6,847
Average recorded investment:
Total impaired loans with no related allowance recorded	6,733	6,712	6,903	6,644	6,860
Interest Recognized
Total impaired loans with no related allowance recorded	81	75	241	220	298
Residential Junior Lien
Interest Recognized
Total trouble debt restructuring	750		750		812
Residential Junior Lien | Class of Financing Receivable
Recorded investment:
Total impaired loans with no related allowance	762		762		978
Unpaid principal balance:
Total impaired loans with no related allowance recorded	774		774		1,017
Average recorded investment:
Total impaired loans with no related allowance recorded	765	1,012	975	1,011	1,011
Interest Recognized
Total impaired loans with no related allowance recorded	11	12	35	39	52
Consumer
Interest Recognized
Total trouble debt restructuring	1,266		1,266		1,269
Consumer | Class of Financing Receivable
Recorded investment:
Total impaired loans with no related allowance	1,318		1,318		1,295
Unpaid principal balance:
Total impaired loans with no related allowance recorded	1,319		1,319		1,295
Average recorded investment:
Total impaired loans with no related allowance recorded	1,317	1,229	1,278	1,191	1,214
Interest Recognized
Total impaired loans with no related allowance recorded	2		2	1	1
Foreign
Interest Recognized
Total trouble debt restructuring	355		355		360
Foreign | Class of Financing Receivable
Recorded investment:
Total impaired loans with no related allowance	355		355		746
Unpaid principal balance:
Total impaired loans with no related allowance recorded	355		355		746
Average recorded investment:
Total impaired loans with no related allowance recorded	759	752	753	752	751
Interest Recognized
Total impaired loans with no related allowance recorded	$ 4	$ 4	$ 12	$ 12	$ 16