Allowance for Probable Loan Losses (Impaired Loans) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Interest Recognized
Total trouble debt restructuring	$ 21,382		$ 21,382		$ 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	2,836		$ 2,836		4,828
Total impaired loans with no related allowance	40,534		40,534		42,577
Unpaid principal balance:
Total impaired loans with an allowance recorded	5,132		5,132		5,981
Total impaired loans with no related allowance recorded	69,238		69,238		74,677
Related Allowance	1,628		1,628		961
Average recorded investment:
Total impaired loans with an allowance recorded	3,233	$ 9,376	3,758	$ 9,839	5,821
Total impaired loans with no related allowance recorded	41,202	31,509	42,196	38,093	40,450
Interest Recognized
Total impaired loans with an allowance recorded		22		48
Total impaired loans with no related allowance recorded	128	112	250	228	547
Commercial
Interest Recognized
Total trouble debt restructuring	9,866		9,866		10,710
Commercial | Class of Financing Receivable
Recorded investment:
Total impaired loans with related allowance	1,148		1,148
Total impaired loans with no related allowance	19,963		19,963		21,412
Unpaid principal balance:
Total impaired loans with an allowance recorded	2,288		2,288
Total impaired loans with no related allowance recorded	47,689		47,689		50,737
Related Allowance	436		436
Average recorded investment:
Total impaired loans with an allowance recorded	1,152	3,453	1,158	3,735
Total impaired loans with no related allowance recorded	20,005	10,837	20,463	15,944	19,354
Interest Recognized
Total impaired loans with no related allowance recorded	1	1	1	2	3
Commercial Real Estate: Other Construction and Land Development | Class of Financing Receivable
Recorded investment:
Total impaired loans with related allowance	363		363		1,958
Total impaired loans with no related allowance	2,442		2,442		2,818
Unpaid principal balance:
Total impaired loans with an allowance recorded	382		382		1,971
Total impaired loans with no related allowance recorded	2,553		2,553		4,419
Related Allowance	329		329		371
Average recorded investment:
Total impaired loans with an allowance recorded	767	162	1,033	163	2,512
Total impaired loans with no related allowance recorded	2,482	5,155	2,610	5,726	2,336
Interest Recognized
Total impaired loans with no related allowance recorded		20		45	67
Commercial Real Estate: Farmland & Commercial
Interest Recognized
Total trouble debt restructuring	3,044		3,044		3,086
Commercial Real Estate: Farmland & Commercial | Class of Financing Receivable
Recorded investment:
Total impaired loans with related allowance	1,325		1,325		2,808
Total impaired loans with no related allowance	7,994		7,994		8,002
Unpaid principal balance:
Total impaired loans with an allowance recorded	2,462		2,462		3,948
Total impaired loans with no related allowance recorded	8,760		8,760		9,054
Related Allowance	863		863		546
Average recorded investment:
Total impaired loans with an allowance recorded	1,314	5,761	1,567	5,941	3,247
Total impaired loans with no related allowance recorded	8,532	5,709	8,774	6,649	8,523
Interest Recognized
Total impaired loans with an allowance recorded		22		48
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					62
Total impaired loans with no related allowance	515		515		552
Unpaid principal balance:
Total impaired loans with an allowance recorded					62
Total impaired loans with no related allowance recorded	532		532		562
Related Allowance					44
Average recorded investment:
Total impaired loans with an allowance recorded					62
Total impaired loans with no related allowance recorded	521	301	530	235	401
Residential First Lien
Interest Recognized
Total trouble debt restructuring	6,228		6,228		6,181
Residential First Lien | Class of Financing Receivable
Recorded investment:
Total impaired loans with no related allowance	6,749		6,749		6,774
Unpaid principal balance:
Total impaired loans with no related allowance recorded	6,812		6,812		6,847
Average recorded investment:
Total impaired loans with no related allowance recorded	6,774	6,610	6,874	6,594	6,860
Interest Recognized
Total impaired loans with no related allowance recorded	81	74	159	145	298
Residential Junior Lien
Interest Recognized
Total trouble debt restructuring	793		793		812
Residential Junior Lien | Class of Financing Receivable
Recorded investment:
Total impaired loans with no related allowance	965		965		978
Unpaid principal balance:
Total impaired loans with no related allowance recorded	985		985		1,017
Average recorded investment:
Total impaired loans with no related allowance recorded	977	1,008	982	1,022	1,011
Interest Recognized
Total impaired loans with no related allowance recorded	12	12	23	27	52
Consumer
Interest Recognized
Total trouble debt restructuring	1,091		1,091		1,269
Consumer | Class of Financing Receivable
Recorded investment:
Total impaired loans with no related allowance	1,152		1,152		1,295
Unpaid principal balance:
Total impaired loans with no related allowance recorded	1,153		1,153		1,295
Average recorded investment:
Total impaired loans with no related allowance recorded	1,156	1,134	1,212	1,172	1,214
Interest Recognized
Total impaired loans with no related allowance recorded		1	1	1	1
Foreign
Interest Recognized
Total trouble debt restructuring	360		360		360
Foreign | Class of Financing Receivable
Recorded investment:
Total impaired loans with no related allowance	754		754		746
Unpaid principal balance:
Total impaired loans with no related allowance recorded	754		754		746
Average recorded investment:
Total impaired loans with no related allowance recorded	755	755	751	751	751
Interest Recognized
Total impaired loans with no related allowance recorded	$ 4	$ 4	$ 8	$ 8	$ 16