Loans and Allowance for Loan Leases - Internal Risk Ratings (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	$ 4,011,169	$ 3,861,707
Pass & Watch
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	3,827,787	3,727,182
Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	78,863	47,629
Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	100,431	81,413
Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	4,088	5,483
Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Commercial
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	1,355,723	1,356,817
Commercial | Pass & Watch
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	1,234,385	1,258,952
Commercial | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	49,459	27,069
Commercial | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	71,879	70,796
Commercial | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Commercial | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Energy
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	398,797	408,573
Energy | Pass & Watch
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	370,614	392,233
Energy | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	12,628	9,460
Energy | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	12,294	2,340
Energy | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	3,261	4,540
Energy | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	1,084,684	1,024,041
Commercial real estate | Pass & Watch
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	1,058,543	1,007,921
Commercial real estate | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	14,985	9,311
Commercial real estate | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	10,329	5,866
Commercial real estate | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	827	943
Commercial real estate | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Construction and land development
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	625,145	628,418
Construction and land development | Pass & Watch
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	625,145	628,418
Construction and land development | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Construction and land development | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Construction and land development | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Construction and land development | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Residential real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	503,702	398,695
Residential real estate | Pass & Watch
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	496,236	394,495
Residential real estate | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	1,791	1,789
Residential real estate | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	5,675	2,411
Residential real estate | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Residential real estate | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	43,118	45,163
Consumer | Pass & Watch
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	42,864	45,163
Consumer | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	254	0
Consumer | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	0	0
Consumer | Loss
Financing Receivable, Credit Quality Indicator [Line Items]
Gross loans	$ 0	$ 0