Loans and Allowance for Credit Losses - Age Analysis of Past Due Loans, Excluding Loans Acquired, Segregated by Class of Loans (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Financing Receivable, Past Due [Line Items]
Total Past Due	$ 88,011	$ 93,989
Current	14,518,889	14,331,715
Total Loans	14,606,900	14,425,704
90 Days Past Due & Accruing	494	856
Gross 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	32,166	57,289
90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	55,845	36,700
Consumer
Financing Receivable, Past Due [Line Items]
Total Past Due	4,284	7,108
Current	394,088	446,889
Total Loans	398,372	453,997
90 Days Past Due & Accruing	263	259
Consumer | Gross 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	3,298	5,732
Consumer | 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	986	1,376
Consumer | Credit cards
Financing Receivable, Past Due [Line Items]
Total Past Due	934	1,489
Current	183,414	203,313
Total Loans	184,348	204,802
90 Days Past Due & Accruing	262	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
Consumer | Other consumer
Financing Receivable, Past Due [Line Items]
Total Past Due	3,350	5,619
Current	210,674	243,576
Total Loans	214,024	249,195
90 Days Past Due & Accruing	1	0
Consumer | Other consumer | Gross 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	2,626	4,884
Consumer | Other consumer | 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	724	735
Real Estate
Financing Receivable, Past Due [Line Items]
Total Past Due	61,611	61,072
Current	10,472,176	10,960,868
Total Loans	10,533,787	11,021,940
90 Days Past Due & Accruing	50	597
Real Estate | Gross 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	22,044	39,755
Real Estate | 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	39,567	21,317
Real Estate | Construction and development
Financing Receivable, Past Due [Line Items]
Total Past Due	8,272	6,870
Current	2,001,984	2,241,803
Total Loans	2,010,256	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	3,574	5,792
Real Estate | Construction and development | 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	4,698	1,078
Real Estate | Single family residential
Financing Receivable, Past Due [Line Items]
Total Past Due	29,194	40,107
Current	2,177,893	2,374,646
Total Loans	2,207,087	2,414,753
90 Days Past Due & Accruing	0	597
Real Estate | Single family residential | Gross 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	13,805	26,318
Real Estate | Single family residential | 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	15,389	13,789
Real Estate | Other commercial
Financing Receivable, Past Due [Line Items]
Total Past Due	24,145	14,095
Current	6,292,299	6,344,419
Total Loans	6,316,444	6,358,514
90 Days Past Due & Accruing	50	0
Real Estate | Other commercial | Gross 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	4,665	7,645
Real Estate | Other commercial | 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	19,480	6,450
Commercial
Financing Receivable, Past Due [Line Items]
Total Past Due	22,116	25,809
Current	3,233,815	2,616,835
Total Loans	3,255,931	2,642,644
90 Days Past Due & Accruing	181	0
Commercial | Gross 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	6,824	11,802
Commercial | 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	15,292	14,007
Commercial | Commercial
Financing Receivable, Past Due [Line Items]
Total Past Due	21,304	24,130
Current	3,016,912	2,426,989
Total Loans	3,038,216	2,451,119
90 Days Past Due & Accruing	180	0
Commercial | Commercial | Gross 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	6,413	10,579
Commercial | Commercial | 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	14,891	13,551
Commercial | Agricultural
Financing Receivable, Past Due [Line Items]
Total Past Due	812	1,679
Current	216,903	189,846
Total Loans	217,715	191,525
90 Days Past Due & Accruing	1	0
Commercial | Agricultural | Gross 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	411	1,223
Commercial | Agricultural | 90 Days or More Past Due
Financing Receivable, Past Due [Line Items]
Total Past Due	401	456
Other
Financing Receivable, Past Due [Line Items]
Total Past Due	0	0
Current	418,810	307,123
Total Loans	418,810	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