Loans and Allowance for Loan Leases - Impaired Loan Interest Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Financing Receivable, Impaired [Line Items]
Total interest income recognized	$ 1,195	$ 1,702
Total average impaired loans	124,013	109,914
Commercial
Financing Receivable, Impaired [Line Items]
Total interest income recognized	910	1,285
Total average impaired loans	86,626	77,698
Energy
Financing Receivable, Impaired [Line Items]
Total interest income recognized	122	152
Total average impaired loans	16,976	15,986
Commercial real estate
Financing Receivable, Impaired [Line Items]
Total interest income recognized	123	254
Total average impaired loans	14,927	14,073
Construction and land development
Financing Receivable, Impaired [Line Items]
Total interest income recognized	0	0
Total average impaired loans	0	0
Residential real estate
Financing Receivable, Impaired [Line Items]
Total interest income recognized	40	11
Total average impaired loans	5,230	2,157
Consumer
Financing Receivable, Impaired [Line Items]
Total interest income recognized	0	0
Total average impaired loans	$ 254	$ 0