Loans Held for Investment (LHFI) and Allowance for Loan Losses, LHFI - Aging Analysis of Past Due and Nonaccrual LHFI by Loan Type (Details) - USD ($) $ in Thousands		Sep. 30, 2019	Dec. 31, 2018
Financing Receivable, Recorded Investment, Aging [Abstract]
Total Past Due		$ 11,327	$ 17,691
Nonaccrual		59,030	61,624
Current Loans		9,153,311	8,756,553
Total LHFI		9,223,668	8,835,868
Past Due 30 to 59 Days [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total Past Due		8,486	14,383
Past Due 60 to 89 Days [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total Past Due		1,963	2,452
Past Due 90 Days or More [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total Past Due	[1]	878	856
Construction, Land Development and Other Land [Member] | Loans Secured by Real Estate [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total Past Due		279	284
Nonaccrual		1,144	2,218
Current Loans		1,134,576	1,054,099
Total LHFI		1,135,999	1,056,601
Construction, Land Development and Other Land [Member] | Loans Secured by Real Estate [Member] | Past Due 30 to 59 Days [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total Past Due		240	284
Construction, Land Development and Other Land [Member] | Loans Secured by Real Estate [Member] | Past Due 60 to 89 Days [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total Past Due		39
Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total Past Due		5,183	10,869
Nonaccrual		18,598	14,718
Current Loans		1,796,674	1,799,905
Total LHFI		1,820,455	1,825,492
Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member] | Past Due 30 to 59 Days [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total Past Due		3,928	8,600
Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member] | Past Due 60 to 89 Days [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total Past Due		715	1,700
Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member] | Past Due 90 Days or More [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total Past Due	[1]	540	569
Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total Past Due		2,218	1,887
Nonaccrual		7,757	9,621
Current Loans		2,432,333	2,209,406
Total LHFI		2,442,308	2,220,914
Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member] | Past Due 30 to 59 Days [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total Past Due		1,453	1,887
Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member] | Past Due 60 to 89 Days [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total Past Due		765
Other Real Estate Secured [Member] | Loans Secured by Real Estate [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total Past Due		177	296
Nonaccrual		1,067	927
Current Loans		667,423	542,597
Total LHFI		668,667	543,820
Other Real Estate Secured [Member] | Loans Secured by Real Estate [Member] | Past Due 30 to 59 Days [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total Past Due		177	197
Other Real Estate Secured [Member] | Loans Secured by Real Estate [Member] | Past Due 60 to 89 Days [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total Past Due			99
Commercial and Industrial Loans [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total Past Due		1,194	1,646
Nonaccrual		25,313	23,938
Current Loans		1,464,860	1,513,131
Total LHFI		1,491,367	1,538,715
Commercial and Industrial Loans [Member] | Past Due 30 to 59 Days [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total Past Due		973	1,346
Commercial and Industrial Loans [Member] | Past Due 60 to 89 Days [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total Past Due		181	300
Commercial and Industrial Loans [Member] | Past Due 90 Days or More [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total Past Due	[1]	40
Consumer Loans [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total Past Due		2,079	2,440
Nonaccrual		110	205
Current Loans		174,705	179,803
Total LHFI		176,894	182,448
Consumer Loans [Member] | Past Due 30 to 59 Days [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total Past Due		1,554	1,800
Consumer Loans [Member] | Past Due 60 to 89 Days [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total Past Due		227	353
Consumer Loans [Member] | Past Due 90 Days or More [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total Past Due	[1]	298	287
State and Other Political Subdivision Loans [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total Past Due			186
Nonaccrual		4,078	8,595
Current Loans		974,378	965,037
Total LHFI		978,456	973,818
State and Other Political Subdivision Loans [Member] | Past Due 30 to 59 Days [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total Past Due			186
Other Loans [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total Past Due		197	83
Nonaccrual		963	1,402
Current Loans		508,362	492,575
Total LHFI		509,522	494,060
Other Loans [Member] | Past Due 30 to 59 Days [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total Past Due		161	$ 83
Other Loans [Member] | Past Due 60 to 89 Days [Member]
Financing Receivable, Recorded Investment, Aging [Abstract]
Total Past Due		$ 36

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