Allowance for Probable Loan Losses (Impaired Loans) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Interest Recognized
Total trouble debt restructuring	$ 8,074		$ 8,158
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	$ 25,922		1,698
Total impaired loans with no related allowance	30,127		22,202
Unpaid principal balance:
Total impaired loans with an allowance recorded	26,851		2,330
Total impaired loans with no related allowance recorded	31,166		23,284
Related Allowance	10,282		772
Average recorded investment:
Total impaired loans with an allowance recorded	25,942	$ 16,964	1,882
Total impaired loans with no related allowance recorded	30,619	50,280	65,192
Interest Recognized
Total impaired loans with no related allowance recorded	90	93	368
Commercial
Interest Recognized
Total trouble debt restructuring	34		35
Commercial \| Class of Financing Receivable
Recorded investment:
Total impaired loans with related allowance	1,325		1,563
Total impaired loans with no related allowance	17,283		7,616
Unpaid principal balance:
Total impaired loans with an allowance recorded	1,908		2,161
Total impaired loans with no related allowance recorded	17,420		7,730
Related Allowance	666		656
Average recorded investment:
Total impaired loans with an allowance recorded	1,327	1,717	1,741
Total impaired loans with no related allowance recorded	17,370	16,820	16,194
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	134		135
Total impaired loans with no related allowance	1,925		1,957
Unpaid principal balance:
Total impaired loans with an allowance recorded	169		169
Total impaired loans with no related allowance recorded	2,167		2,205
Related Allowance	116		116
Average recorded investment:
Total impaired loans with an allowance recorded	135	144	141
Total impaired loans with no related allowance recorded	1,924	2,284	2,151
Commercial Real Estate: Farmland and Commercial \| Class of Financing Receivable
Recorded investment:
Total impaired loans with related allowance	24,463
Total impaired loans with no related allowance	1,928		3,509
Unpaid principal balance:
Total impaired loans with an allowance recorded	24,774
Total impaired loans with no related allowance recorded	2,493		4,031
Related Allowance	9,500
Average recorded investment:
Total impaired loans with an allowance recorded	24,480	15,103
Total impaired loans with no related allowance recorded	2,305	21,488	36,632
Commercial Real Estate: Multifamily \| Class of Financing Receivable
Recorded investment:
Total impaired loans with no related allowance	507		507
Unpaid principal balance:
Total impaired loans with no related allowance recorded	539		538
Average recorded investment:
Total impaired loans with no related allowance recorded	506	471	565
Residential First Lien
Interest Recognized
Total trouble debt restructuring	5,951		5,947
Residential First Lien \| Class of Financing Receivable
Recorded investment:
Total impaired loans with no related allowance	6,216		6,244
Unpaid principal balance:
Total impaired loans with no related allowance recorded	6,268		6,386
Average recorded investment:
Total impaired loans with no related allowance recorded	6,239	6,960	7,136
Interest Recognized
Total impaired loans with no related allowance recorded	75	77	305
Residential Junior Lien
Interest Recognized
Total trouble debt restructuring	743		730
Residential Junior Lien \| Class of Financing Receivable
Recorded investment:
Total impaired loans with no related allowance	908		901
Unpaid principal balance:
Total impaired loans with no related allowance recorded	918		911
Average recorded investment:
Total impaired loans with no related allowance recorded	912	708	976
Interest Recognized
Total impaired loans with no related allowance recorded	11	10	44
Consumer
Interest Recognized
Total trouble debt restructuring	1,060		1,153
Consumer \| Class of Financing Receivable
Recorded investment:
Total impaired loans with no related allowance	1,074		1,175
Unpaid principal balance:
Total impaired loans with no related allowance recorded	1,075		1,190
Average recorded investment:
Total impaired loans with no related allowance recorded	1,075	1,208	1,211
Interest Recognized
Total impaired loans with no related allowance recorded		1	2
Foreign
Interest Recognized
Total trouble debt restructuring	286		293
Foreign \| Class of Financing Receivable
Recorded investment:
Total impaired loans with no related allowance	286		293
Unpaid principal balance:
Total impaired loans with no related allowance recorded	286		293
Average recorded investment:
Total impaired loans with no related allowance recorded	288	341	327
Interest Recognized
Total impaired loans with no related allowance recorded	$ 3	$ 4	$ 14

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The amount of the outstanding recorded investment related to financing receivables that have been modified by troubled debt restructurings.

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Amount of allowance for credit losses related to recorded investment.

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Amount of interest income, recognized on accrual basis, on financing receivables that are recognized as impaired, for which there is no related allowance for credit losses.

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Amount of average investment of impaired financing receivables for which there is a related allowance for credit losses.

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Threshold period past due for financing receivable to write off as uncollectible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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