Loans Held for Investment (LHFI) and Allowance for Loan Losses, LHFI - Change in Allowance for Loan Losses (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period				$ 79,290
Provision for loan losses, LHFI		$ 3,039	$ 8,673	7,136	$ 15,801
Balance at end of period		83,226		83,226
Allowance for Loan Losses, LHFI [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period		80,399	83,566	79,290	76,733
Transfers	[1]		772		1,554
Loans charged-off		(2,892)	(7,017)	(9,862)	(12,980)
Recoveries		2,680	2,880	6,662	7,766
Net (charge-offs) recoveries		(212)	(4,137)	(3,200)	(5,214)
Provision for loan losses, LHFI		3,039	8,673	7,136	15,801
Balance at end of period		83,226	88,874	83,226	88,874
Allowance for Loan Losses, LHFI [Member] | Construction, Land Development and Other Land [Member] | Loans Secured by Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period				7,390	7,865
Transfers	[1]				584
Loans charged-off				(35)	(123)
Recoveries				807	1,039
Provision for loan losses, LHFI				(461)	(951)
Balance at end of period		7,701	8,414	7,701	8,414
Allowance for Loan Losses, LHFI [Member] | Secured by 1-4 Family Residential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period				8,641	10,874
Transfers	[1]				182
Loans charged-off				(310)	(1,435)
Recoveries				530	439
Provision for loan losses, LHFI				(417)	(752)
Balance at end of period		8,444	9,308	8,444	9,308
Allowance for Loan Losses, LHFI [Member] | Secured by Nonfarm, Nonresidential Properties [Member] | Loans Secured by Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period				22,376	23,428
Transfers	[1]				446
Loans charged-off				(261)	(1,117)
Recoveries				285	120
Provision for loan losses, LHFI				2,802	1,982
Balance at end of period		25,202	24,859	25,202	24,859
Allowance for Loan Losses, LHFI [Member] | Other Real Estate Secured [Member] | Loans Secured by Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period				3,450	2,790
Transfers	[1]				291
Recoveries				22	17
Provision for loan losses, LHFI				82	(149)
Balance at end of period		3,554	2,949	3,554	2,949
Allowance for Loan Losses, LHFI [Member] | Commercial and Industrial Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period				27,359	22,851
Transfers	[1]				46
Loans charged-off				(3,090)	(4,658)
Recoveries				980	2,285
Provision for loan losses, LHFI				3,618	12,793
Balance at end of period		28,867	33,317	28,867	33,317
Allowance for Loan Losses, LHFI [Member] | Consumer Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period				2,890	3,470
Transfers	[1]				5
Loans charged-off				(1,712)	(1,585)
Recoveries				1,432	1,545
Provision for loan losses, LHFI				702	(41)
Balance at end of period		3,312	3,394	3,312	3,394
Allowance for Loan Losses, LHFI [Member] | State and Other Political Subdivision Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period				990	789
Provision for loan losses, LHFI				(518)	404
Balance at end of period		472	1,193	472	1,193
Allowance for Loan Losses, LHFI [Member] | Other Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period				6,194	4,666
Loans charged-off				(4,454)	(4,062)
Recoveries				2,606	2,321
Provision for loan losses, LHFI				1,328	2,515
Balance at end of period		$ 5,674	$ 5,440	$ 5,674	$ 5,440

[1] The allowance for loan losses balance related to the remaining loans acquired in the Bay Bank merger, which were transferred from acquired loans to LHFI during the second quarter of 2018, and the remaining loans acquired in the Heritage and Reliance mergers, which were transferred from acquired loans to LHFI during the third quarter of 2018.