Loans and Allowance for Loan Leases - Loan Aging Analysis (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Financing Receivable, Past Due [Line Items]
Total Past Due	$ 39,420	$ 48,834
Current	3,971,749	3,812,873
Total Loans Receivable	4,011,169	3,861,707
Loans >= 90 Days and Accruing	0	4,591
30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	12,934	6,292
60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	6,604	530
90 Days or More
Financing Receivable, Past Due [Line Items]
Total Past Due	19,882	42,012
Commercial
Financing Receivable, Past Due [Line Items]
Total Past Due	15,666	32,278
Current	1,340,057	1,324,539
Total Loans Receivable	1,355,723	1,356,817
Loans >= 90 Days and Accruing	0	37
Commercial | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	5,135	1,091
Commercial | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	469	276
Commercial | 90 Days or More
Financing Receivable, Past Due [Line Items]
Total Past Due	10,062	30,911
Energy
Financing Receivable, Past Due [Line Items]
Total Past Due	8,926	6,933
Current	389,871	401,640
Total Loans Receivable	398,797	408,573
Loans >= 90 Days and Accruing	0	53
Energy | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	0	2,340
Energy | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	5,612	0
Energy | 90 Days or More
Financing Receivable, Past Due [Line Items]
Total Past Due	3,314	4,593
Commercial real estate
Financing Receivable, Past Due [Line Items]
Total Past Due	8,678	4,905
Current	1,076,006	1,019,136
Total Loans Receivable	1,084,684	1,024,041
Loans >= 90 Days and Accruing	0	4,501
Commercial real estate | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	4,092	316
Commercial real estate | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Commercial real estate | 90 Days or More
Financing Receivable, Past Due [Line Items]
Total Past Due	4,586	4,589
Construction and land development
Financing Receivable, Past Due [Line Items]
Total Past Due	0	196
Current	625,145	628,222
Total Loans Receivable	625,145	628,418
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	0	196
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	5,896	4,266
Current	497,806	394,429
Total Loans Receivable	503,702	398,695
Loans >= 90 Days and Accruing	0	0
Residential real estate | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	3,707	2,347
Residential real estate | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	269	0
Residential real estate | 90 Days or More
Financing Receivable, Past Due [Line Items]
Total Past Due	1,920	1,919
Consumer
Financing Receivable, Past Due [Line Items]
Total Past Due	254	256
Current	42,864	44,907
Total Loans Receivable	43,118	45,163
Loans >= 90 Days and Accruing	0	0
Consumer | 30-59 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	0	2
Consumer | 60-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	254	254
Consumer | 90 Days or More
Financing Receivable, Past Due [Line Items]
Total Past Due	$ 0	$ 0