Loans and Allowance for Loan Losses - Category of Loans (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 4,429,241		$ 3,861,707
Less: Allowance for loan losses	71,185	$ 51,458	56,896	$ 42,852	$ 40,001	$ 37,826
Less: Net deferred loan fees and costs	16,017		9,463
Net loans	4,342,039		3,795,348
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	1,284,919		1,356,817
Less: Allowance for loan losses	26,543	21,129	35,864	22,975	20,506	16,584
Energy
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	390,346		408,573
Less: Allowance for loan losses	17,372	7,599	6,565	7,300	7,090	10,262
Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	1,141,277		1,024,041
Less: Allowance for loan losses	16,899	12,623	8,085	7,533	7,471	6,755
Construction and land development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	661,691		628,418
Less: Allowance for loan losses	5,019	5,021	3,516	2,602	2,585	2,475
Residential real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	536,270		398,695
Less: Allowance for loan losses	4,868	4,687	2,546	2,138	2,047	1,464
Paycheck Protection Program (“PPP”)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	369,022		0
Less: Allowance for loan losses	0	0	0	0	0	0
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	45,716		45,163
Less: Allowance for loan losses	$ 484	$ 399	$ 320	$ 304	$ 302	$ 286