Loans and Allowance for Credit Losses - Loans by Credit Risk Rating, Segregated by Class of Loans (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
2020 (YTD)	$ 67,517
2019	22,117
2018	31,928
2017	14,898
2016	11,533
2015 and Prior	26,416
Lines of Credit (“LOC”) Amortized Cost Basis	165,036
LOC Converted to Term Loans Amortized Cost Basis	36,653
Total	376,098	$ 14,425,704
Risk Rate 1-4
Financing Receivable, Credit Quality Indicator [Line Items]
Total		14,113,275
Risk Rate 5
Financing Receivable, Credit Quality Indicator [Line Items]
Total		102,618
Risk Rate 6
Financing Receivable, Credit Quality Indicator [Line Items]
Total		209,421
Risk Rate 7
Financing Receivable, Credit Quality Indicator [Line Items]
Total		173
Risk Rate 8
Financing Receivable, Credit Quality Indicator [Line Items]
Total		217
Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Total		454,496
Consumer | Risk Rate 1-4
Financing Receivable, Credit Quality Indicator [Line Items]
Total		451,829
Consumer | Risk Rate 5
Financing Receivable, Credit Quality Indicator [Line Items]
Total		0
Consumer | Risk Rate 6
Financing Receivable, Credit Quality Indicator [Line Items]
Total		2,667
Consumer | Risk Rate 7
Financing Receivable, Credit Quality Indicator [Line Items]
Total		0
Consumer | Risk Rate 8
Financing Receivable, Credit Quality Indicator [Line Items]
Total		0
Consumer | Credit cards
Financing Receivable, Credit Quality Indicator [Line Items]
2020 (YTD)	0
2019	0
2018	0
2017	0
2016	0
2015 and Prior	0
Lines of Credit (“LOC”) Amortized Cost Basis	934
LOC Converted to Term Loans Amortized Cost Basis	0
Total	934	204,802
Consumer | Credit cards | Risk Rate 1-4
Financing Receivable, Credit Quality Indicator [Line Items]
Total		204,161
Consumer | Credit cards | Risk Rate 5
Financing Receivable, Credit Quality Indicator [Line Items]
Total		0
Consumer | Credit cards | Risk Rate 6
Financing Receivable, Credit Quality Indicator [Line Items]
Total		641
Consumer | Credit cards | Risk Rate 7
Financing Receivable, Credit Quality Indicator [Line Items]
Total		0
Consumer | Credit cards | Risk Rate 8
Financing Receivable, Credit Quality Indicator [Line Items]
Total		0
Consumer | Other consumer
Financing Receivable, Credit Quality Indicator [Line Items]
2020 (YTD)	120
2019	556
2018	494
2017	867
2016	768
2015 and Prior	100
Lines of Credit (“LOC”) Amortized Cost Basis	445
LOC Converted to Term Loans Amortized Cost Basis	0
Total	3,350	249,694
Consumer | Other consumer | Risk Rate 1-4
Financing Receivable, Credit Quality Indicator [Line Items]
Total		247,668
Consumer | Other consumer | Risk Rate 5
Financing Receivable, Credit Quality Indicator [Line Items]
Total		0
Consumer | Other consumer | Risk Rate 6
Financing Receivable, Credit Quality Indicator [Line Items]
Total		2,026
Consumer | Other consumer | Risk Rate 7
Financing Receivable, Credit Quality Indicator [Line Items]
Total		0
Consumer | Other consumer | Risk Rate 8
Financing Receivable, Credit Quality Indicator [Line Items]
Total		0
Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
Total		10,884,524
Real Estate | Risk Rate 1-4
Financing Receivable, Credit Quality Indicator [Line Items]
Total		10,691,728
Real Estate | Risk Rate 5
Financing Receivable, Credit Quality Indicator [Line Items]
Total		75,864
Real Estate | Risk Rate 6
Financing Receivable, Credit Quality Indicator [Line Items]
Total		116,765
Real Estate | Risk Rate 7
Financing Receivable, Credit Quality Indicator [Line Items]
Total		130
Real Estate | Risk Rate 8
Financing Receivable, Credit Quality Indicator [Line Items]
Total		37
Real Estate | Construction and development
Financing Receivable, Credit Quality Indicator [Line Items]
2020 (YTD)	241
2019	3,031
2018	442
2017	2,144
2016	442
2015 and Prior	1,082
Lines of Credit (“LOC”) Amortized Cost Basis	850
LOC Converted to Term Loans Amortized Cost Basis	799
Total	9,031	2,236,861
Real Estate | Construction and development | Risk Rate 1-4
Financing Receivable, Credit Quality Indicator [Line Items]
Total		2,229,019
Real Estate | Construction and development | Risk Rate 5
Financing Receivable, Credit Quality Indicator [Line Items]
2020 (YTD)	0
2019	43
2018	17
2017	1,947
2016	20
2015 and Prior	0
Lines of Credit (“LOC”) Amortized Cost Basis	0
LOC Converted to Term Loans Amortized Cost Basis	0
Total	2,027	70
Real Estate | Construction and development | Risk Rate 6
Financing Receivable, Credit Quality Indicator [Line Items]
2020 (YTD)	241
2019	2,988
2018	425
2017	197
2016	422
2015 and Prior	1,082
Lines of Credit (“LOC”) Amortized Cost Basis	850
LOC Converted to Term Loans Amortized Cost Basis	799
Total	7,004	7,735
Real Estate | Construction and development | Risk Rate 7
Financing Receivable, Credit Quality Indicator [Line Items]
2020 (YTD)	0
2019	0
2018	0
2017	0
2016	0
2015 and Prior	0
Lines of Credit (“LOC”) Amortized Cost Basis	0
LOC Converted to Term Loans Amortized Cost Basis	0
Total	0	0
Real Estate | Construction and development | Risk Rate 8
Financing Receivable, Credit Quality Indicator [Line Items]
Total		37
Real Estate | Single family residential
Financing Receivable, Credit Quality Indicator [Line Items]
2020 (YTD)	321
2019	4,476
2018	4,896
2017	3,793
2016	3,314
2015 and Prior	8,375
Lines of Credit (“LOC”) Amortized Cost Basis	4,019
LOC Converted to Term Loans Amortized Cost Basis	0
Total	29,194	2,442,064
Real Estate | Single family residential | Risk Rate 1-4
Financing Receivable, Credit Quality Indicator [Line Items]
Total		2,394,284
Real Estate | Single family residential | Risk Rate 5
Financing Receivable, Credit Quality Indicator [Line Items]
Total		6,049
Real Estate | Single family residential | Risk Rate 6
Financing Receivable, Credit Quality Indicator [Line Items]
Total		41,601
Real Estate | Single family residential | Risk Rate 7
Financing Receivable, Credit Quality Indicator [Line Items]
Total		130
Real Estate | Single family residential | Risk Rate 8
Financing Receivable, Credit Quality Indicator [Line Items]
Total		0
Real Estate | Other commercial
Financing Receivable, Credit Quality Indicator [Line Items]
2020 (YTD)	53,849
2019	8,433
2018	20,525
2017	5,460
2016	5,459
2015 and Prior	16,093
Lines of Credit (“LOC”) Amortized Cost Basis	57,100
LOC Converted to Term Loans Amortized Cost Basis	34,994
Total	201,913	6,205,599
Real Estate | Other commercial | Risk Rate 1-4
Financing Receivable, Credit Quality Indicator [Line Items]
Total		6,068,425
Real Estate | Other commercial | Risk Rate 5
Financing Receivable, Credit Quality Indicator [Line Items]
2020 (YTD)	18,591
2019	2,425
2018	11,530
2017	1,049
2016	1,349
2015 and Prior	2,591
Lines of Credit (“LOC”) Amortized Cost Basis	18,027
LOC Converted to Term Loans Amortized Cost Basis	24,917
Total	80,479	69,745
Real Estate | Other commercial | Risk Rate 6
Financing Receivable, Credit Quality Indicator [Line Items]
2020 (YTD)	35,258
2019	6,008
2018	8,995
2017	4,411
2016	4,110
2015 and Prior	13,502
Lines of Credit (“LOC”) Amortized Cost Basis	39,073
LOC Converted to Term Loans Amortized Cost Basis	10,077
Total	121,434	67,429
Real Estate | Other commercial | Risk Rate 7
Financing Receivable, Credit Quality Indicator [Line Items]
2020 (YTD)	0
2019	0
2018	0
2017	0
2016	0
2015 and Prior	0
Lines of Credit (“LOC”) Amortized Cost Basis	0
LOC Converted to Term Loans Amortized Cost Basis	0
Total	0	0
Real Estate | Other commercial | Risk Rate 8
Financing Receivable, Credit Quality Indicator [Line Items]
Total		0
Commercial
Financing Receivable, Credit Quality Indicator [Line Items]
Total		2,810,970
Commercial | Risk Rate 1-4
Financing Receivable, Credit Quality Indicator [Line Items]
Total		2,694,004
Commercial | Risk Rate 5
Financing Receivable, Credit Quality Indicator [Line Items]
Total		26,754
Commercial | Risk Rate 6
Financing Receivable, Credit Quality Indicator [Line Items]
Total		89,989
Commercial | Risk Rate 7
Financing Receivable, Credit Quality Indicator [Line Items]
Total		43
Commercial | Risk Rate 8
Financing Receivable, Credit Quality Indicator [Line Items]
Total		180
Commercial | Commercial
Financing Receivable, Credit Quality Indicator [Line Items]
2020 (YTD)	12,965
2019	5,423
2018	5,363
2017	2,160
2016	1,401
2015 and Prior	755
Lines of Credit (“LOC”) Amortized Cost Basis	101,588
LOC Converted to Term Loans Amortized Cost Basis	860
Total	130,515	2,495,516
Commercial | Commercial | Risk Rate 1-4
Financing Receivable, Credit Quality Indicator [Line Items]
Total		2,384,263
Commercial | Commercial | Risk Rate 5
Financing Receivable, Credit Quality Indicator [Line Items]
2020 (YTD)	3,377
2019	325
2018	478
2017	136
2016	233
2015 and Prior	58
Lines of Credit (“LOC”) Amortized Cost Basis	46,465
LOC Converted to Term Loans Amortized Cost Basis	0
Total	51,072	26,713
Commercial | Commercial | Risk Rate 6
Financing Receivable, Credit Quality Indicator [Line Items]
2020 (YTD)	9,588
2019	5,098
2018	4,885
2017	2,024
2016	1,168
2015 and Prior	697
Lines of Credit (“LOC”) Amortized Cost Basis	55,123
LOC Converted to Term Loans Amortized Cost Basis	860
Total	79,443	84,317
Commercial | Commercial | Risk Rate 7
Financing Receivable, Credit Quality Indicator [Line Items]
2020 (YTD)	0
2019	0
2018	0
2017	0
2016	0
2015 and Prior	0
Lines of Credit (“LOC”) Amortized Cost Basis	0
LOC Converted to Term Loans Amortized Cost Basis	0
Total	0	43
Commercial | Commercial | Risk Rate 8
Financing Receivable, Credit Quality Indicator [Line Items]
Total		180
Commercial | Agricultural
Financing Receivable, Credit Quality Indicator [Line Items]
2020 (YTD)	21
2019	198
2018	208
2017	474
2016	149
2015 and Prior	11
Lines of Credit (“LOC”) Amortized Cost Basis	100
LOC Converted to Term Loans Amortized Cost Basis	0
Total	1,161	315,454
Commercial | Agricultural | Risk Rate 1-4
Financing Receivable, Credit Quality Indicator [Line Items]
Total		309,741
Commercial | Agricultural | Risk Rate 5
Financing Receivable, Credit Quality Indicator [Line Items]
2020 (YTD)	0
2019	86
2018	15
2017	379
2016	0
2015 and Prior	0
Lines of Credit (“LOC”) Amortized Cost Basis	37
LOC Converted to Term Loans Amortized Cost Basis	0
Total	517	41
Commercial | Agricultural | Risk Rate 6
Financing Receivable, Credit Quality Indicator [Line Items]
2020 (YTD)	21
2019	112
2018	193
2017	95
2016	149
2015 and Prior	11
Lines of Credit (“LOC”) Amortized Cost Basis	63
LOC Converted to Term Loans Amortized Cost Basis	0
Total	644	5,672
Commercial | Agricultural | Risk Rate 7
Financing Receivable, Credit Quality Indicator [Line Items]
2020 (YTD)	0
2019	0
2018	0
2017	0
2016	0
2015 and Prior	0
Lines of Credit (“LOC”) Amortized Cost Basis	0
LOC Converted to Term Loans Amortized Cost Basis	0
Total	0	0
Commercial | Agricultural | Risk Rate 8
Financing Receivable, Credit Quality Indicator [Line Items]
Total		0
Other
Financing Receivable, Credit Quality Indicator [Line Items]
Total		275,714
Other | Risk Rate 1-4
Financing Receivable, Credit Quality Indicator [Line Items]
Total		275,714
Other | Risk Rate 5
Financing Receivable, Credit Quality Indicator [Line Items]
Total		0
Other | Risk Rate 6
Financing Receivable, Credit Quality Indicator [Line Items]
Total		0
Other | Risk Rate 7
Financing Receivable, Credit Quality Indicator [Line Items]
Total		0
Other | Risk Rate 8
Financing Receivable, Credit Quality Indicator [Line Items]
Total		$ 0
30-89 days past due | Consumer | Credit cards
Financing Receivable, Credit Quality Indicator [Line Items]
2020 (YTD)	0
2019	0
2018	0
2017	0
2016	0
2015 and Prior	0
Lines of Credit (“LOC”) Amortized Cost Basis	672
LOC Converted to Term Loans Amortized Cost Basis	0
Total	672
30-89 days past due | Consumer | Other consumer
Financing Receivable, Credit Quality Indicator [Line Items]
2020 (YTD)	89
2019	446
2018	387
2017	576
2016	633
2015 and Prior	71
Lines of Credit (“LOC”) Amortized Cost Basis	424
LOC Converted to Term Loans Amortized Cost Basis	0
Total	2,626
30-89 days past due | Real Estate | Single family residential
Financing Receivable, Credit Quality Indicator [Line Items]
2020 (YTD)	321
2019	2,074
2018	1,980
2017	1,415
2016	1,796
2015 and Prior	4,553
Lines of Credit (“LOC”) Amortized Cost Basis	1,666
LOC Converted to Term Loans Amortized Cost Basis	0
Total	13,805
90 Days or More Past Due | Consumer | Credit cards
Financing Receivable, Credit Quality Indicator [Line Items]
2020 (YTD)	0
2019	0
2018	0
2017	0
2016	0
2015 and Prior	0
Lines of Credit (“LOC”) Amortized Cost Basis	262
LOC Converted to Term Loans Amortized Cost Basis	0
Total	262
90 Days or More Past Due | Consumer | Other consumer
Financing Receivable, Credit Quality Indicator [Line Items]
2020 (YTD)	31
2019	110
2018	107
2017	291
2016	135
2015 and Prior	29
Lines of Credit (“LOC”) Amortized Cost Basis	21
LOC Converted to Term Loans Amortized Cost Basis	0
Total	724
90 Days or More Past Due | Real Estate | Single family residential
Financing Receivable, Credit Quality Indicator [Line Items]
2020 (YTD)	0
2019	2,402
2018	2,916
2017	2,378
2016	1,518
2015 and Prior	3,822
Lines of Credit (“LOC”) Amortized Cost Basis	2,353
LOC Converted to Term Loans Amortized Cost Basis	0
Total	$ 15,389