Loans and Allowance for Credit Losses - Age Analysis of Past Due Loans, Excluding Loans Acquired, Segregated by Class of Loans (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	$ 74,260	$ 93,989
Current	12,826,637	14,331,715
Total Loans	12,900,897	14,425,704
90 Days Past Due & Accruing	578	856
Gross 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	39,072	57,289
90 Days or More Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	35,188	36,700
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	4,037	7,108
Current	387,187	447,388
Total Loans	391,224	454,496
90 Days Past Due & Accruing	269	259
Consumer | Gross 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	3,479	5,732
Consumer | 90 Days or More Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	558	1,376
Consumer | Credit cards
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	964	1,489
Current	179,390	203,313
Total Loans	180,354	204,802
90 Days Past Due & Accruing	256	259
Consumer | Credit cards | Gross 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	708	848
Consumer | Credit cards | 90 Days or More Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	256	641
Total Loans	256
Consumer | Other consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	3,073	5,619
Current	207,797	244,075
Total Loans	210,870	249,694
90 Days Past Due & Accruing	13	0
Consumer | Other consumer | Gross 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	2,771	4,884
Consumer | Other consumer | 90 Days or More Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	302	735
Total Loans	302
Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	54,775	61,072
Current	9,169,016	10,823,452
Total Loans	9,223,791	10,884,524
90 Days Past Due & Accruing	253	597
Real Estate | Gross 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	27,761	39,755
Real Estate | 90 Days or More Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	27,014	21,317
Real Estate | Construction and development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	4,464	6,870
Current	1,591,791	2,229,991
Total Loans	1,596,255	2,236,861
90 Days Past Due & Accruing	0	0
Real Estate | Construction and development | Gross 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	1,375	5,792
Real Estate | Construction and development | 90 Days or More Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	3,089	1,078
Real Estate | Single family residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	38,065	40,107
Current	1,842,608	2,401,957
Total Loans	1,880,673	2,442,064
90 Days Past Due & Accruing	253	597
Real Estate | Single family residential | Gross 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	23,726	26,318
Real Estate | Single family residential | 90 Days or More Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	14,339	13,789
Total Loans	14,339
Real Estate | Other commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	12,246	14,095
Current	5,734,617	6,191,504
Total Loans	5,746,863	6,205,599
90 Days Past Due & Accruing	0	0
Real Estate | Other commercial | Gross 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	2,660	7,645
Real Estate | Other commercial | 90 Days or More Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	9,586	6,450
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	15,356	25,809
Current	2,734,935	2,785,161
Total Loans	2,750,291	2,810,970
90 Days Past Due & Accruing	56	0
Commercial | Gross 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	7,740	11,802
Commercial | 90 Days or More Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	7,616	14,007
Commercial | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	14,943	24,130
Current	2,559,443	2,471,386
Total Loans	2,574,386	2,495,516
90 Days Past Due & Accruing	56	0
Commercial | Commercial | Gross 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	7,514	10,579
Commercial | Commercial | 90 Days or More Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	7,429	13,551
Commercial | Agricultural
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	413	1,679
Current	175,492	313,775
Total Loans	175,905	315,454
90 Days Past Due & Accruing	0	0
Commercial | Agricultural | Gross 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	226	1,223
Commercial | Agricultural | 90 Days or More Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	187	456
Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	92	0
Current	535,499	275,714
Total Loans	535,591	275,714
90 Days Past Due & Accruing	0	$ 0
Other | Gross 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	92
Other | 90 Days or More Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	$ 0