Loans and Allowance for Loan Leases - Loan Aging Analysis (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Financing Receivable, Past Due [Line Items]
Total Past Due	$ 73,251	$ 19,289
Current	3,565,442	3,050,090
Total Loans Receivable	3,638,693	3,069,379
Loans 90 Days and Accruing	642	0
30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	61,941	3,062
60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	2,785	619
90 Days or More
Financing Receivable, Past Due [Line Items]
Total Past Due	8,525	15,608
Commercial
Financing Receivable, Past Due [Line Items]
Total Past Due	45,925	5,177
Current	1,266,722	1,129,237
Total Loans Receivable	1,312,647	1,134,414
Loans 90 Days and Accruing	0	0
Commercial | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	42,039	1,040
Commercial | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	2,785	0
Commercial | 90 Days or More
Financing Receivable, Past Due [Line Items]
Total Past Due	1,101	4,137
Energy
Financing Receivable, Past Due [Line Items]
Total Past Due	12,441	11,212
Current	383,691	347,071
Total Loans Receivable	396,132	358,283
Loans 90 Days and Accruing	642	0
Energy | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	7,122	1,994
Energy | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Energy | 90 Days or More
Financing Receivable, Past Due [Line Items]
Total Past Due	5,319	9,218
Commercial real estate
Financing Receivable, Past Due [Line Items]
Total Past Due	410	2,678
Current	992,743	843,883
Total Loans Receivable	993,153	846,561
Loans 90 Days and Accruing	0	0
Commercial real estate | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	317	0
Commercial real estate | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	0	425
Commercial real estate | 90 Days or More
Financing Receivable, Past Due [Line Items]
Total Past Due	93	2,253
Construction and land development
Financing Receivable, Past Due [Line Items]
Total Past Due	12,345	0
Current	515,237	440,032
Total Loans Receivable	527,582	440,032
Loans 90 Days and Accruing	0	0
Construction and land development | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	12,345	0
Construction and land development | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Construction and land development | 90 Days or More
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Residential real estate
Financing Receivable, Past Due [Line Items]
Total Past Due	2,080	222
Current	363,355	246,053
Total Loans Receivable	365,435	246,275
Loans 90 Days and Accruing	0	0
Residential real estate | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	68	28
Residential real estate | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	0	194
Residential real estate | 90 Days or More
Financing Receivable, Past Due [Line Items]
Total Past Due	2,012	0
Equity lines of credit
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Current	22,192	20,286
Total Loans Receivable	22,192	20,286
Loans 90 Days and Accruing	0	0
Equity lines of credit | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Equity lines of credit | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Equity lines of credit | 90 Days or More
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Consumer installment
Financing Receivable, Past Due [Line Items]
Total Past Due	50	0
Current	21,502	23,528
Total Loans Receivable	21,552	23,528
Loans 90 Days and Accruing	0	0
Consumer installment | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	50	0
Consumer installment | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Consumer installment | 90 Days or More
Financing Receivable, Past Due [Line Items]
Total Past Due	$ 0	$ 0