Loans and Allowance for Loan Losses - Loans by Credit Risk Rating, Segregated by Class of Loans (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
2020 (YTD)	$ 3,861
2019	63,661
2018	38,126
2017	33,137
2016	22,153
2015 and Prior	37,205
Lines of Credit Amortized Cost Basis	138,142
Lines of Credit Converted to Term Loans Amortized Cost Basis	1,989
Total	338,274	$ 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 Amortized Cost Basis	1,191
Lines of Credit Converted to Term Loans Amortized Cost Basis	0
Total	1,191	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)	47
2019	1,256
2018	733
2017	1,634
2016	1,385
2015 and Prior	311
Lines of Credit Amortized Cost Basis	306
Lines of Credit Converted to Term Loans Amortized Cost Basis	0
Total	5,672	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)	0
2019	3,614
2018	639
2017	2,163
2016	453
2015 and Prior	626
Lines of Credit Amortized Cost Basis	2,401
Lines of Credit Converted to Term Loans Amortized Cost Basis	209
Total	10,105	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	45
2018	19
2017	1,957
2016	21
2015 and Prior	0
Lines of Credit Amortized Cost Basis	0
Lines of Credit Converted to Term Loans Amortized Cost Basis	0
Total	2,042	70
Real Estate | Construction and development | Risk rate 6
Financing Receivable, Credit Quality Indicator [Line Items]
2020 (YTD)	0
2019	3,569
2018	620
2017	206
2016	432
2015 and Prior	626
Lines of Credit Amortized Cost Basis	2,401
Lines of Credit Converted to Term Loans Amortized Cost Basis	209
Total	8,063	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 Amortized Cost Basis	0
Lines of Credit 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)	33
2019	6,787
2018	10,242
2017	7,042
2016	6,839
2015 and Prior	11,981
Lines of Credit Amortized Cost Basis	3,557
Lines of Credit Converted to Term Loans Amortized Cost Basis	0
Total	46,481	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)	3,765
2019	39,688
2018	19,327
2017	5,354
2016	11,705
2015 and Prior	23,438
Lines of Credit Amortized Cost Basis	50,029
Lines of Credit Converted to Term Loans Amortized Cost Basis	1,037
Total	154,343	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)	1,037
2019	23,628
2018	2,116
2017	985
2016	4,845
2015 and Prior	3,379
Lines of Credit Amortized Cost Basis	12,477
Lines of Credit Converted to Term Loans Amortized Cost Basis	0
Total	48,467	69,745
Real Estate | Other commercial | Risk rate 6
Financing Receivable, Credit Quality Indicator [Line Items]
2020 (YTD)	2,728
2019	16,060
2018	17,211
2017	4,369
2016	6,860
2015 and Prior	20,059
Lines of Credit Amortized Cost Basis	37,552
Lines of Credit Converted to Term Loans Amortized Cost Basis	1,037
Total	105,876	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 Amortized Cost Basis	0
Lines of Credit 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)	16
2019	12,134
2018	7,008
2017	16,750
2016	1,671
2015 and Prior	841
Lines of Credit Amortized Cost Basis	80,411
Lines of Credit Converted to Term Loans Amortized Cost Basis	743
Total	119,574	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)	0
2019	209
2018	14
2017	270
2016	407
2015 and Prior	77
Lines of Credit Amortized Cost Basis	25,418
Lines of Credit Converted to Term Loans Amortized Cost Basis	0
Total	26,395	26,713
Commercial | Commercial | Risk rate 6
Financing Receivable, Credit Quality Indicator [Line Items]
2020 (YTD)	16
2019	11,925
2018	6,994
2017	16,480
2016	1,264
2015 and Prior	764
Lines of Credit Amortized Cost Basis	54,993
Lines of Credit Converted to Term Loans Amortized Cost Basis	743
Total	93,179	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 Amortized Cost Basis	0
Lines of Credit 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)	0
2019	182
2018	177
2017	194
2016	100
2015 and Prior	8
Lines of Credit Amortized Cost Basis	247
Lines of Credit Converted to Term Loans Amortized Cost Basis	0
Total	908	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	67
2018	0
2017	25
2016	12
2015 and Prior	0
Lines of Credit Amortized Cost Basis	17
Lines of Credit Converted to Term Loans Amortized Cost Basis	0
Total	121	41
Commercial | Agricultural | Risk rate 6
Financing Receivable, Credit Quality Indicator [Line Items]
2020 (YTD)	0
2019	115
2018	177
2017	169
2016	88
2015 and Prior	8
Lines of Credit Amortized Cost Basis	230
Lines of Credit Converted to Term Loans Amortized Cost Basis	0
Total	787	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 Amortized Cost Basis	0
Lines of Credit 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 Amortized Cost Basis	866
Lines of Credit Converted to Term Loans Amortized Cost Basis	0
Total	866
30-89 days past due | Consumer | Other consumer
Financing Receivable, Credit Quality Indicator [Line Items]
2020 (YTD)	47
2019	1,109
2018	636
2017	1,188
2016	1,182
2015 and Prior	251
Lines of Credit Amortized Cost Basis	128
Lines of Credit Converted to Term Loans Amortized Cost Basis	0
Total	4,541
30-89 days past due | Real Estate | Single family residential
Financing Receivable, Credit Quality Indicator [Line Items]
2020 (YTD)	33
2019	4,672
2018	6,168
2017	4,648
2016	5,010
2015 and Prior	8,233
Lines of Credit Amortized Cost Basis	2,471
Lines of Credit Converted to Term Loans Amortized Cost Basis	0
Total	31,235
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 Amortized Cost Basis	325
Lines of Credit Converted to Term Loans Amortized Cost Basis	0
Total	325
90 Days or More Past Due | Consumer | Other consumer
Financing Receivable, Credit Quality Indicator [Line Items]
2020 (YTD)	0
2019	147
2018	97
2017	446
2016	203
2015 and Prior	60
Lines of Credit Amortized Cost Basis	178
Lines of Credit Converted to Term Loans Amortized Cost Basis	0
Total	1,131
90 Days or More Past Due | Real Estate | Single family residential
Financing Receivable, Credit Quality Indicator [Line Items]
2020 (YTD)	0
2019	2,115
2018	4,074
2017	2,394
2016	1,829
2015 and Prior	3,748
Lines of Credit Amortized Cost Basis	1,086
Lines of Credit Converted to Term Loans Amortized Cost Basis	0
Total	$ 15,246