Loans and Allowance for Credit Losses - Age Analysis of Past Due Loans, Excluding Loans Acquired, Segregated by Class of Loans (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Financing Receivable, Past Due [Line Items]
Total Past Due	$ 64,439	$ 93,989
Current	13,953,003	14,331,715
Total	14,017,442	14,425,704
90 Days Past Due & Accruing	174	856
Gross 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	28,701	57,289
90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	35,738	36,700
Consumer
Financing Receivable, Past Due [Line Items]
Total Past Due	3,627	7,108
Current	359,989	446,889
Total	363,616	453,997
90 Days Past Due & Accruing	101	259
Consumer | Gross 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	2,974	5,732
Consumer | 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	653	1,376
Consumer | Credit cards
Financing Receivable, Past Due [Line Items]
Total Past Due	934	1,489
Current	171,946	203,313
Total	172,880	204,802
90 Days Past Due & Accruing	95	259
Consumer | Credit cards | Gross 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	672	848
Consumer | Credit cards | 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	262	641
Total	262
Consumer | Other consumer
Financing Receivable, Past Due [Line Items]
Total Past Due	2,693	5,619
Current	188,043	243,576
Total	190,736	249,195
90 Days Past Due & Accruing	6	0
Consumer | Other consumer | Gross 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	2,302	4,884
Consumer | Other consumer | 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	391	735
Total	391
Real Estate
Financing Receivable, Past Due [Line Items]
Total Past Due	47,386	61,072
Current	9,935,867	10,960,868
Total	9,983,253	11,021,940
90 Days Past Due & Accruing	66	597
Real Estate | Gross 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	18,453	39,755
Real Estate | 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	28,933	21,317
Real Estate | Construction and development
Financing Receivable, Past Due [Line Items]
Total Past Due	3,694	6,870
Current	1,849,666	2,241,803
Total	1,853,360	2,248,673
90 Days Past Due & Accruing	0	0
Real Estate | Construction and development | Gross 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	1,168	5,792
Real Estate | Construction and development | 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	2,526	1,078
Real Estate | Single family residential
Financing Receivable, Past Due [Line Items]
Total Past Due	26,692	40,107
Current	1,970,378	2,374,646
Total	1,997,070	2,414,753
90 Days Past Due & Accruing	64	597
Real Estate | Single family residential | Gross 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	12,170	26,318
Real Estate | Single family residential | 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	14,522	13,789
Total	14,523
Real Estate | Other commercial
Financing Receivable, Past Due [Line Items]
Total Past Due	17,000	14,095
Current	6,115,823	6,344,419
Total	6,132,823	6,358,514
90 Days Past Due & Accruing	2	0
Real Estate | Other commercial | Gross 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	5,115	7,645
Real Estate | Other commercial | 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	11,885	6,450
Commercial
Financing Receivable, Past Due [Line Items]
Total Past Due	13,426	25,809
Current	3,136,059	2,616,835
Total	3,149,485	2,642,644
90 Days Past Due & Accruing	7	0
Commercial | Gross 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	7,274	11,802
Commercial | 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	6,152	14,007
Commercial | Commercial
Financing Receivable, Past Due [Line Items]
Total Past Due	12,808	24,130
Current	2,894,990	2,426,989
Total	2,907,798	2,451,119
90 Days Past Due & Accruing	7	0
Commercial | Commercial | Gross 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	6,984	10,579
Commercial | Commercial | 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	5,824	13,551
Commercial | Agricultural
Financing Receivable, Past Due [Line Items]
Total Past Due	618	1,679
Current	241,069	189,846
Total	241,687	191,525
90 Days Past Due & Accruing	0	0
Commercial | Agricultural | Gross 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	290	1,223
Commercial | Agricultural | 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	328	456
Other
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Current	521,088	307,123
Total	521,088	307,123
90 Days Past Due & Accruing	0	$ 0
Other | Gross 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	0
Other | 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	$ 0