Allowance for Probable Loan Losses (Impaired Loans) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Interest Recognized
Total trouble debt restructuring	$ 14,874		$ 14,874		$ 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	1,544		$ 1,544		1,894
Total impaired loans with no related allowance	33,896		33,896		61,310
Unpaid principal balance:
Total impaired loans with an allowance recorded	2,161		2,161		2,336
Total impaired loans with no related allowance recorded	62,218		62,218		89,664
Related Allowance	576		576		434
Average recorded investment:
Total impaired loans with an allowance recorded	1,747	$ 3,233	1,805	$ 3,758	1,985
Total impaired loans with no related allowance recorded	64,903	41,202	65,399	42,196	40,450
Interest Recognized
Total impaired loans with no related allowance recorded	94	128	184	250	547
Commercial
Interest Recognized
Total trouble debt restructuring	6,742		6,742		6,910
Commercial | Class of Financing Receivable
Recorded investment:
Total impaired loans with related allowance	1,402		1,402
Total impaired loans with no related allowance	16,531		16,531		16,646
Unpaid principal balance:
Total impaired loans with an allowance recorded	1,992		1,992
Total impaired loans with no related allowance recorded	43,980		43,980		44,095
Related Allowance	460		460
Average recorded investment:
Total impaired loans with an allowance recorded	1,604	1,152	1,661	1,158
Total impaired loans with no related allowance recorded	16,594	20,005	16,738	20,463	19,354
Interest Recognized
Total impaired loans with no related allowance recorded	1	1	1	1	3
Commercial Real Estate:Other Construction and Land Development | Class of Financing Receivable
Recorded investment:
Total impaired loans with related allowance	142		142		1,300
Total impaired loans with no related allowance	2,024		2,024		2,310
Unpaid principal balance:
Total impaired loans with an allowance recorded	169		169		1,577
Total impaired loans with no related allowance recorded	2,230		2,230		2,455
Related Allowance	116		116		300
Average recorded investment:
Total impaired loans with an allowance recorded	143	767	144	1,033	1,346
Total impaired loans with no related allowance recorded	2,148	2,482	2,216	2,610	2,336
Interest Recognized
Total impaired loans with no related allowance recorded					67
Commercial Real Estate: Farmland and Commercial | Class of Financing Receivable
Recorded investment:
Total impaired loans with related allowance					145
Total impaired loans with no related allowance	6,020		6,020		32,675
Unpaid principal balance:
Total impaired loans with an allowance recorded					169
Total impaired loans with no related allowance recorded	6,520		6,520		33,275
Related Allowance					116
Average recorded investment:
Total impaired loans with an allowance recorded		1,314		1,567	150
Total impaired loans with no related allowance recorded	36,790	8,532	36,686	8,774	8,523
Interest Recognized
Total impaired loans with no related allowance recorded		30		58	110
Commercial Real Estate: Multifamily | Class of Financing Receivable
Recorded investment:
Total impaired loans with related allowance					449
Total impaired loans with no related allowance	664		664		476
Unpaid principal balance:
Total impaired loans with an allowance recorded					590
Total impaired loans with no related allowance recorded	696		696		505
Related Allowance					18
Average recorded investment:
Total impaired loans with an allowance recorded					489
Total impaired loans with no related allowance recorded	664	521	568	530	401
Residential First Lien
Interest Recognized
Total trouble debt restructuring	5,902		5,902		6,140
Residential First Lien | Class of Financing Receivable
Recorded investment:
Total impaired loans with no related allowance	6,387		6,387		6,852
Unpaid principal balance:
Total impaired loans with no related allowance recorded	6,508		6,508		6,968
Average recorded investment:
Total impaired loans with no related allowance recorded	6,426	6,774	6,864	6,874	6,860
Interest Recognized
Total impaired loans with no related allowance recorded	76	81	152	159	298
Residential Junior Lien
Interest Recognized
Total trouble debt restructuring	799		799		712
Residential Junior Lien | Class of Financing Receivable
Recorded investment:
Total impaired loans with no related allowance	799		799		723
Unpaid principal balance:
Total impaired loans with no related allowance recorded	810		810		736
Average recorded investment:
Total impaired loans with no related allowance recorded	803	977	811	982	1,011
Interest Recognized
Total impaired loans with no related allowance recorded	12	12	22	23	52
Consumer
Interest Recognized
Total trouble debt restructuring	1,101		1,101		1,237
Consumer | Class of Financing Receivable
Recorded investment:
Total impaired loans with no related allowance	1,141		1,141		1,281
Unpaid principal balance:
Total impaired loans with no related allowance recorded	1,144		1,144		1,283
Average recorded investment:
Total impaired loans with no related allowance recorded	1,145	1,156	1,179	1,212	1,214
Interest Recognized
Total impaired loans with no related allowance recorded	1		2	1	1
Foreign
Interest Recognized
Total trouble debt restructuring	330		330		347
Foreign | Class of Financing Receivable
Recorded investment:
Total impaired loans with no related allowance	330		330		347
Unpaid principal balance:
Total impaired loans with no related allowance recorded	330		330		347
Average recorded investment:
Total impaired loans with no related allowance recorded	333	755	337	751	751
Interest Recognized
Total impaired loans with no related allowance recorded	$ 4	$ 4	$ 7	$ 8	$ 16