Allowance for Probable Loan Losses (Impaired Loans) (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Interest Recognized
Total trouble debt restructuring	$ 21,708		$ 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	$ 3,516		4,828
Total impaired loans with no related allowance	39,497		42,577
Unpaid principal balance:
Total impaired loans with an allowance recorded	5,801		5,981
Total impaired loans with no related allowance recorded	68,282		74,677
Related Allowance	1,504		961
Average recorded investment:
Total impaired loans with an allowance recorded	4,346	$ 10,302	5,821
Total impaired loans with no related allowance recorded	40,286	44,438	40,450
Interest Recognized
Total impaired loans with an allowance recorded		26
Total impaired loans with no related allowance recorded	124	116	547
Commercial
Interest Recognized
Total trouble debt restructuring	10,113		10,710
Commercial | Class of Financing Receivable
Recorded investment:
Total impaired loans with related allowance	1,159
Total impaired loans with no related allowance	20,177		21,412
Unpaid principal balance:
Total impaired loans with an allowance recorded	2,288
Total impaired loans with no related allowance recorded	47,901		50,737
Related Allowance	278
Average recorded investment:
Total impaired loans with an allowance recorded	1,165	4,017
Total impaired loans with no related allowance recorded	20,789	21,051	19,354
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	970		1,958
Total impaired loans with no related allowance	2,617		2,818
Unpaid principal balance:
Total impaired loans with an allowance recorded	986		1,971
Total impaired loans with no related allowance recorded	2,705		4,419
Related Allowance	319		371
Average recorded investment:
Total impaired loans with an allowance recorded	1,299	165	2,512
Total impaired loans with no related allowance recorded	2,683	6,296	2,336
Interest Recognized
Total impaired loans with no related allowance recorded		25	67
Commercial Real Estate: Farmland & Commercial
Interest Recognized
Total trouble debt restructuring	3,055		3,086
Commercial Real Estate: Farmland & Commercial | Class of Financing Receivable
Recorded investment:
Total impaired loans with related allowance	1,325		2,808
Total impaired loans with no related allowance	6,672		8,002
Unpaid principal balance:
Total impaired loans with an allowance recorded	2,465		3,948
Total impaired loans with no related allowance recorded	7,529		9,054
Related Allowance	863		546
Average recorded investment:
Total impaired loans with an allowance recorded	1,820	6,120	3,247
Total impaired loans with no related allowance recorded	6,751	7,590	8,523
Interest Recognized
Total impaired loans with an allowance recorded		26
Total impaired loans with no related allowance recorded	28		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	532		552
Unpaid principal balance:
Total impaired loans with an allowance recorded			62
Total impaired loans with no related allowance recorded	547		562
Related Allowance			44
Average recorded investment:
Total impaired loans with an allowance recorded			62
Total impaired loans with no related allowance recorded	539	168	401
Residential First Lien
Interest Recognized
Total trouble debt restructuring	6,166		6,181
Residential First Lien | Class of Financing Receivable
Recorded investment:
Total impaired loans with related allowance	62
Total impaired loans with no related allowance	6,528		6,774
Unpaid principal balance:
Total impaired loans with an allowance recorded	62
Total impaired loans with no related allowance recorded	6,605		6,847
Related Allowance	44
Average recorded investment:
Total impaired loans with an allowance recorded	62
Total impaired loans with no related allowance recorded	6,558	6,340	6,860
Interest Recognized
Total impaired loans with no related allowance recorded	79	71	298
Residential Junior Lien
Interest Recognized
Total trouble debt restructuring	803		812
Residential Junior Lien | Class of Financing Receivable
Recorded investment:
Total impaired loans with no related allowance	968		978
Unpaid principal balance:
Total impaired loans with no related allowance recorded	992		1,017
Average recorded investment:
Total impaired loans with no related allowance recorded	972	1,036	1,011
Interest Recognized
Total impaired loans with no related allowance recorded	12	15	52
Consumer
Interest Recognized
Total trouble debt restructuring	1,204		1,269
Consumer | Class of Financing Receivable
Recorded investment:
Total impaired loans with no related allowance	1,246		1,295
Unpaid principal balance:
Total impaired loans with no related allowance recorded	1,246		1,295
Average recorded investment:
Total impaired loans with no related allowance recorded	1,246	1,210	1,214
Interest Recognized
Total impaired loans with no related allowance recorded			1
Foreign
Interest Recognized
Total trouble debt restructuring	367		360
Foreign | Class of Financing Receivable
Recorded investment:
Total impaired loans with no related allowance	757		746
Unpaid principal balance:
Total impaired loans with no related allowance recorded	757		746
Average recorded investment:
Total impaired loans with no related allowance recorded	748	747	751
Interest Recognized
Total impaired loans with no related allowance recorded	$ 4	$ 4	$ 16