Loans and Allowance for Credit Losses - Loan Portfolio by Categories (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Loans and Leases Receivable Disclosure [Line Items]
Total loans	$ 14,017,442	$ 14,425,704
Consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans	363,616	453,997
Consumer | Credit cards
Loans and Leases Receivable Disclosure [Line Items]
Total loans	172,880	204,802
Consumer | Other consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans	190,736	249,195
Real Estate
Loans and Leases Receivable Disclosure [Line Items]
Total loans	9,983,253	11,021,940
Real Estate | Construction and development
Loans and Leases Receivable Disclosure [Line Items]
Total loans	1,853,360	2,248,673
Real Estate | Single family residential
Loans and Leases Receivable Disclosure [Line Items]
Total loans	1,997,070	2,414,753
Real Estate | Other commercial
Loans and Leases Receivable Disclosure [Line Items]
Total loans	6,132,823	6,358,514
Commercial
Loans and Leases Receivable Disclosure [Line Items]
Total loans	3,149,485	2,642,644
Commercial | Commercial
Loans and Leases Receivable Disclosure [Line Items]
Total loans	2,907,798	2,451,119
Commercial | Agricultural
Loans and Leases Receivable Disclosure [Line Items]
Total loans	241,687	191,525
Other
Loans and Leases Receivable Disclosure [Line Items]
Total loans	$ 521,088	$ 307,123