Loans Held for Investment (LHFI) and Allowance for Loan Losses, LHFI - Summary of LHFI by Loan Type and Credit Quality Indicator (Details) - USD ($) $ in Thousands		Mar. 31, 2020	Dec. 31, 2019
Financing Receivable [Abstract]
Financing receivable commercial		$ 9,567,920
Current		9,537,117	$ 9,269,775
Financing Receivable, Recorded Investment, Past Due		30,803	12,627
Nonaccrual		52,992	53,226
Past Due 90 Days or More [Member]
Financing Receivable [Abstract]
Financing Receivable, Recorded Investment, Past Due		12,481	642	[1]
Construction, Land Development and Other Land [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Current		485,568	1,161,258
Financing Receivable, Recorded Investment, Past Due		697	636
Nonaccrual		950	897
Construction, Land Development and Other Land [Member] | Loans Secured by Real Estate [Member] | Past Due 90 Days or More [Member]
Financing Receivable [Abstract]
Financing Receivable, Recorded Investment, Past Due		178
Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Current			1,832,698
Financing Receivable, Recorded Investment, Past Due			6,405
Nonaccrual			16,810
Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member] | Past Due 90 Days or More [Member]
Financing Receivable [Abstract]
Financing Receivable, Recorded Investment, Past Due	[1]		211
Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Current		2,572,508	2,465,847
Financing Receivable, Recorded Investment, Past Due		2,914	1,698
Nonaccrual		7,323	7,700
Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member] | Past Due 90 Days or More [Member]
Financing Receivable [Abstract]
Financing Receivable, Recorded Investment, Past Due		283
Other Real Estate Secured [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Current		838,064	723,440
Financing Receivable, Recorded Investment, Past Due		509	8
Nonaccrual		1,076	1,032
Other Real Estate Secured [Member] | Loans Secured by Real Estate [Member] | Past Due 90 Days or More [Member]
Financing Receivable [Abstract]
Financing Receivable, Recorded Investment, Past Due		354
Commercial and Industrial Loans [Member]
Financing Receivable [Abstract]
Current		1,473,312	1,455,453
Financing Receivable, Recorded Investment, Past Due		3,465	668
Nonaccrual		21,156	21,775
Commercial and Industrial Loans [Member] | Past Due 90 Days or More [Member]
Financing Receivable [Abstract]
Financing Receivable, Recorded Investment, Past Due		1,543	39	[1]
Consumer Loans [Member]
Financing Receivable [Abstract]
Current		171,471	172,650
Financing Receivable, Recorded Investment, Past Due		2,207	2,980
Nonaccrual		84	108
Consumer Loans [Member] | Past Due 90 Days or More [Member]
Financing Receivable [Abstract]
Financing Receivable, Recorded Investment, Past Due		297	392	[1]
State and Other Political Subdivision Loans [Member]
Financing Receivable [Abstract]
Current		934,558	963,789
Financing Receivable, Recorded Investment, Past Due		4,079	76
Nonaccrual		4,079	4,079
State and Other Political Subdivision Loans [Member] | Past Due 90 Days or More [Member]
Financing Receivable [Abstract]
Financing Receivable, Recorded Investment, Past Due		4,079
Other Loans [Member]
Financing Receivable [Abstract]
Current			494,640
Financing Receivable, Recorded Investment, Past Due			156
Nonaccrual			825
Commercial LHFI [Member]
Financing Receivable [Abstract]
Financing receivable commercial		7,592,362	7,348,158
Commercial LHFI [Member] | Construction, Land Development and Other Land [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Financing receivable commercial			1,090,914
Commercial LHFI [Member] | Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Financing receivable commercial			123,033
Commercial LHFI [Member] | Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Financing receivable commercial		2,575,400	2,475,090
Commercial LHFI [Member] | Other Real Estate Secured [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Financing receivable commercial			723,785
Commercial LHFI [Member] | Commercial and Industrial Loans [Member]
Financing Receivable [Abstract]
Financing receivable commercial		1,476,777	1,477,896
Commercial LHFI [Member] | State and Other Political Subdivision Loans [Member]
Financing Receivable [Abstract]
Financing receivable commercial		938,637	967,944
Commercial LHFI [Member] | Other Loans [Member]
Financing Receivable [Abstract]
Financing receivable commercial			489,496
Commercial LHFI [Member] | Pass Categories 1-6 [Member]
Financing Receivable [Abstract]
Financing receivable commercial			7,178,617
Commercial LHFI [Member] | Pass Categories 1-6 [Member] | Construction, Land Development and Other Land [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Financing receivable commercial			1,075,146
Commercial LHFI [Member] | Pass Categories 1-6 [Member] | Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Financing receivable commercial			116,592
Commercial LHFI [Member] | Pass Categories 1-6 [Member] | Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Financing receivable commercial		2,520,811	2,430,761
Commercial LHFI [Member] | Pass Categories 1-6 [Member] | Other Real Estate Secured [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Financing receivable commercial			721,238
Commercial LHFI [Member] | Pass Categories 1-6 [Member] | Commercial and Industrial Loans [Member]
Financing Receivable [Abstract]
Financing receivable commercial		1,418,813	1,407,837
Commercial LHFI [Member] | Pass Categories 1-6 [Member] | State and Other Political Subdivision Loans [Member]
Financing Receivable [Abstract]
Financing receivable commercial		928,764	957,948
Commercial LHFI [Member] | Pass Categories 1-6 [Member] | Other Loans [Member]
Financing Receivable [Abstract]
Financing receivable commercial			469,095
Commercial LHFI [Member] | Special Mention Category 7 [Member]
Financing Receivable [Abstract]
Financing receivable commercial			9,049
Commercial LHFI [Member] | Special Mention Category 7 [Member] | Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Financing receivable commercial			45
Commercial LHFI [Member] | Special Mention Category 7 [Member] | Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Financing receivable commercial		5,566
Commercial LHFI [Member] | Special Mention Category 7 [Member] | Commercial and Industrial Loans [Member]
Financing Receivable [Abstract]
Financing receivable commercial			909
Commercial LHFI [Member] | Special Mention Category 7 [Member] | State and Other Political Subdivision Loans [Member]
Financing Receivable [Abstract]
Financing receivable commercial		4,650	4,650
Commercial LHFI [Member] | Special Mention Category 7 [Member] | Other Loans [Member]
Financing Receivable [Abstract]
Financing receivable commercial			3,445
Commercial LHFI [Member] | Substandard Category 8 [Member]
Financing Receivable [Abstract]
Financing receivable commercial			159,163
Commercial LHFI [Member] | Substandard Category 8 [Member] | Construction, Land Development and Other Land [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Financing receivable commercial			15,726
Commercial LHFI [Member] | Substandard Category 8 [Member] | Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Financing receivable commercial			6,355
Commercial LHFI [Member] | Substandard Category 8 [Member] | Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Financing receivable commercial		48,622	44,001
Commercial LHFI [Member] | Substandard Category 8 [Member] | Other Real Estate Secured [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Financing receivable commercial			2,547
Commercial LHFI [Member] | Substandard Category 8 [Member] | Commercial and Industrial Loans [Member]
Financing Receivable [Abstract]
Financing receivable commercial		57,126	68,262
Commercial LHFI [Member] | Substandard Category 8 [Member] | State and Other Political Subdivision Loans [Member]
Financing Receivable [Abstract]
Financing receivable commercial		5,223	5,346
Commercial LHFI [Member] | Substandard Category 8 [Member] | Other Loans [Member]
Financing Receivable [Abstract]
Financing receivable commercial			16,926
Commercial LHFI [Member] | Doubtful Category 9 [Member]
Financing Receivable [Abstract]
Financing receivable commercial			1,329
Commercial LHFI [Member] | Doubtful Category 9 [Member] | Construction, Land Development and Other Land [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Financing receivable commercial			42
Commercial LHFI [Member] | Doubtful Category 9 [Member] | Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Financing receivable commercial			41
Commercial LHFI [Member] | Doubtful Category 9 [Member] | Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Financing receivable commercial		401	328
Commercial LHFI [Member] | Doubtful Category 9 [Member] | Commercial and Industrial Loans [Member]
Financing Receivable [Abstract]
Financing receivable commercial		838	888
Commercial LHFI [Member] | Doubtful Category 9 [Member] | Other Loans [Member]
Financing Receivable [Abstract]
Financing receivable commercial			30
Consumer LHFI [Member]
Financing Receivable [Abstract]
Financing receivable commercial		1,975,558	9,335,628
Current			1,961,967
Nonaccrual			16,057
Subtotal			1,987,470
Consumer LHFI [Member] | Past Due 30-89 Days [Member]
Financing Receivable [Abstract]
Financing Receivable, Recorded Investment, Past Due			8,842
Consumer LHFI [Member] | Past Due 90 Days or More [Member]
Financing Receivable [Abstract]
Financing Receivable, Recorded Investment, Past Due			604
Consumer LHFI [Member] | Construction, Land Development and Other Land [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Financing receivable commercial			1,162,791
Current			71,413
Nonaccrual			132
Subtotal			71,877
Consumer LHFI [Member] | Construction, Land Development and Other Land [Member] | Loans Secured by Real Estate [Member] | Past Due 30-89 Days [Member]
Financing Receivable [Abstract]
Financing Receivable, Recorded Investment, Past Due			332
Consumer LHFI [Member] | Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Financing receivable commercial			1,855,913
Current			1,710,930
Nonaccrual			15,817
Subtotal			1,732,880
Consumer LHFI [Member] | Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member] | Past Due 30-89 Days [Member]
Financing Receivable [Abstract]
Financing Receivable, Recorded Investment, Past Due			5,922
Consumer LHFI [Member] | Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member] | Past Due 90 Days or More [Member]
Financing Receivable [Abstract]
Financing Receivable, Recorded Investment, Past Due			211
Consumer LHFI [Member] | Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Financing receivable commercial		22	2,475,245
Current			155
Subtotal			155
Consumer LHFI [Member] | Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member] | Past Due 30-89 Days [Member]
Financing Receivable [Abstract]
Financing receivable commercial		6
Consumer LHFI [Member] | Other Real Estate Secured [Member] | Loans Secured by Real Estate [Member]
Financing Receivable [Abstract]
Financing receivable commercial			724,480
Current			695
Subtotal			695
Consumer LHFI [Member] | Commercial and Industrial Loans [Member]
Financing Receivable [Abstract]
Financing receivable commercial			1,477,896
Consumer LHFI [Member] | Consumer Loans [Member]
Financing Receivable [Abstract]
Financing receivable commercial		173,678	175,738
Current			172,649
Nonaccrual			108
Subtotal			175,738
Consumer LHFI [Member] | Consumer Loans [Member] | Past Due 30-89 Days [Member]
Financing Receivable [Abstract]
Financing receivable commercial		1,897
Financing Receivable, Recorded Investment, Past Due			2,588
Consumer LHFI [Member] | Consumer Loans [Member] | Past Due 90 Days or More [Member]
Financing Receivable [Abstract]
Financing receivable commercial		$ 296
Financing Receivable, Recorded Investment, Past Due			393
Consumer LHFI [Member] | State and Other Political Subdivision Loans [Member]
Financing Receivable [Abstract]
Financing receivable commercial			967,944
Consumer LHFI [Member] | Other Loans [Member]
Financing Receivable [Abstract]
Financing receivable commercial			495,621
Current			6,125
Subtotal			$ 6,125

[1] Past due 90 days or more but still accruing interest.