Allowance for Probable Loan Losses (Impaired Loans) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Interest Recognized
Total trouble debt restructuring	$ 14,596		$ 14,596		$ 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	1,757		$ 1,757		1,894
Total impaired loans with no related allowance	30,735		30,735		61,310
Unpaid principal balance:
Total impaired loans with an allowance recorded	2,381		2,381		2,336
Total impaired loans with no related allowance recorded	59,022		59,022		89,664
Related Allowance	700		700		434
Average recorded investment:
Total impaired loans with an allowance recorded	1,759	$ 2,864	1,936	$ 3,086	1,985
Total impaired loans with no related allowance recorded	32,938	38,855	65,134	42,273	40,450
Interest Recognized
Total impaired loans with no related allowance recorded	92	120	276	371	547
Receivables from Domestic Operations [Member] | Class of Financing Receivable
Recorded investment:
Total impaired loans with related allowance	138		138
Unpaid principal balance:
Total impaired loans with an allowance recorded	169		169
Related Allowance	116		116
Average recorded investment:
Total impaired loans with an allowance recorded	139		142
Commercial
Interest Recognized
Total trouble debt restructuring	6,440		6,440		6,910
Commercial | Class of Financing Receivable
Recorded investment:
Total impaired loans with no related allowance	15,617		15,617		16,646
Unpaid principal balance:
Total impaired loans with no related allowance recorded	43,068		43,068		44,095
Average recorded investment:
Total impaired loans with an allowance recorded		2,215		2,439
Total impaired loans with no related allowance recorded	15,758	18,547	16,464	18,931	19,354
Interest Recognized
Total impaired loans with no related allowance recorded		1	2	2	3
Commercial Real Estate:Other Construction and Land Development | Class of Financing Receivable
Recorded investment:
Total impaired loans with related allowance	1,619		1,619		1,300
Total impaired loans with no related allowance	1,992		1,992		2,310
Unpaid principal balance:
Total impaired loans with an allowance recorded	2,212		2,212		1,577
Total impaired loans with no related allowance recorded	2,199		2,199		2,455
Related Allowance	584		584		300
Average recorded investment:
Total impaired loans with an allowance recorded	1,620	149	1,794	151	1,346
Total impaired loans with no related allowance recorded	1,994	2,296	2,203	3,456	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	4,157		4,157		32,675
Unpaid principal balance:
Total impaired loans with an allowance recorded					169
Total impaired loans with no related allowance recorded	4,660		4,660		33,275
Related Allowance					116
Average recorded investment:
Total impaired loans with an allowance recorded		500		496	150
Total impaired loans with no related allowance recorded	5,986	7,936	36,633	9,456	8,523
Interest Recognized
Total impaired loans with no related allowance recorded		21		79	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	586		586		476
Unpaid principal balance:
Total impaired loans with an allowance recorded					590
Total impaired loans with no related allowance recorded	617		617		505
Related Allowance					18
Average recorded investment:
Total impaired loans with an allowance recorded					489
Total impaired loans with no related allowance recorded	590	502	575	521	401
Residential First Lien
Interest Recognized
Total trouble debt restructuring	5,976		5,976		6,140
Residential First Lien | Class of Financing Receivable
Recorded investment:
Total impaired loans with no related allowance	6,173		6,173		6,852
Unpaid principal balance:
Total impaired loans with no related allowance recorded	6,257		6,257		6,968
Average recorded investment:
Total impaired loans with no related allowance recorded	6,362	6,733	6,929	6,903	6,860
Interest Recognized
Total impaired loans with no related allowance recorded	76	81	228	241	298
Residential Junior Lien
Interest Recognized
Total trouble debt restructuring	740		740		712
Residential Junior Lien | Class of Financing Receivable
Recorded investment:
Total impaired loans with no related allowance	740		740		723
Unpaid principal balance:
Total impaired loans with no related allowance recorded	748		748		736
Average recorded investment:
Total impaired loans with no related allowance recorded	743	765	807	975	1,011
Interest Recognized
Total impaired loans with no related allowance recorded	11	11	33	35	52
Consumer
Interest Recognized
Total trouble debt restructuring	1,134		1,134		1,237
Consumer | Class of Financing Receivable
Recorded investment:
Total impaired loans with no related allowance	1,164		1,164		1,281
Unpaid principal balance:
Total impaired loans with no related allowance recorded	1,167		1,167		1,283
Average recorded investment:
Total impaired loans with no related allowance recorded	1,187	1,317	1,192	1,278	1,214
Interest Recognized
Total impaired loans with no related allowance recorded	1	2	2	2	1
Foreign
Interest Recognized
Total trouble debt restructuring	306		306		347
Foreign | Class of Financing Receivable
Recorded investment:
Total impaired loans with no related allowance	306		306		347
Unpaid principal balance:
Total impaired loans with no related allowance recorded	306		306		347
Average recorded investment:
Total impaired loans with no related allowance recorded	318	759	331	753	751
Interest Recognized
Total impaired loans with no related allowance recorded	$ 4	$ 4	$ 11	$ 12	$ 16