Loans and Allowance for Loan Losses ("ALLL") - Category of Loans (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 4,493,114		$ 3,861,707
Less: Allowance for loan losses	76,035	$ 71,185	56,896	$ 42,995	$ 42,852	$ 37,826
Less: Net deferred loan fees and costs	15,305		9,463
Net loans	4,401,774		3,795,348
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	1,291,572		1,356,817
Less: Allowance for loan losses	28,203	26,543	35,864	24,811	22,975	16,584
Energy
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	384,181		408,573
Less: Allowance for loan losses	19,540	17,372	6,565	5,377	7,300	10,262
Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	1,195,631		1,024,041
Less: Allowance for loan losses	17,807	16,899	8,085	7,284	7,533	6,755
Construction and land development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	587,617		628,418
Less: Allowance for loan losses	4,489	5,019	3,516	3,016	2,602	2,475
Residential real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	618,082		398,695
Less: Allowance for loan losses	5,494	4,868	2,546	2,220	2,138	1,464
Paycheck Protection Program (“PPP”)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	369,260		0
Less: Allowance for loan losses	0	0	0	0	0	0
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	46,771		45,163
Less: Allowance for loan losses	$ 502	$ 484	$ 320	$ 287	$ 304	$ 286